The embryo as first shewn by Dohrn passes through a Nauplius stage in the brood-pouch, but is hatched, except in the case of the winter eggs of Leptodora, in a form closely resembling the adult.
Copepoda. Amongst the free Copepoda the segmentation and formation of the layers have recently been investigated by Hoek (No. 512). He finds that there is, in both the fresh-water and marine forms studied by him, a centrolecithal segmentation similar to that of Palæmon and Pagurus (vide p. [112]), which might from the surface be supposed to be complete and nearly regular. After the formation of the blastoderm an invagination of some of its cells takes place and is completed in about a quarter of an hour. The opening becomes closed. This invagination is compared by Hoek to the invagination in Astacus, and is believed by him to give rise to the mesenteron. Its point of closing corresponds with the hind end of the embryo. On the ventral surface there appear two transverse furrows dividing the embryo into three segments, and a median longitudinal furrow which does not extend to the front end of the foremost segment. The three pairs of Nauplius appendages and upper lip become subsequently formed as outgrowths from the sides of the ventral blastodermic thickening.
Amongst the parasitic Copepoda there are found two distinct types of segmentation, analogous to those in the Isopoda. In the case of Condracanthus the segmentation is somewhat irregular, but on the type of Eupagurus, etc. (vide p. [112]). In the other group (Anchorella, Clavella, Congericola, Caligus, Lerneopoda) the segmentation nearly resembles the ordinary meroblastic type (vide p. [120]), and is to be explained in the same manner as in the cases of Oniscus and Cymothoa. The first blastodermic cells sometimes appear in a position corresponding with the head end of the embryo (Anchorella), at other times at the hind end (Clavella), and sometimes in the middle of the ventral surface. The dorsal surface of the yolk is always the latest to be inclosed by the blastoderm cells. A larval cuticle similar to that of the Isopoda is formed at the same time as the blastoderm. At the sides of the ventral thickening of the blastoderm there grow out the Nauplius appendages, of which only the first two appear in Anchorella. In Anchorella and Lerneopoda the embryos are not hatched at the Nauplius stage, but after the Nauplius appendages have been formed a fresh cuticle—the Nauplius cuticle—is shed, and within it the embryo develops till it reaches the so-called Cyclops stage (vide p. [490]). The embryo within the egg has its abdomen curved dorsalwards as amongst the Isopoda.
Cirripedia. The segmentation of Balanus and Lepas commences by the segregation of the constituents of the egg into a more protoplasmic portion, and a portion formed mainly of food material. The former separates from the latter as a distinct segment, and then divides into two not quite equal portions. The division of the protoplasmic part of the embryo continues, and the resulting segments grow round the single yolk segment. The point where they finally enclose it is situated on the ventral surface (Lang) at about the position of the mouth (?).
After being enclosed by the protoplasmic cells the yolk divides, and gives rise to a number of cells, which probably supply the material for the walls of the mesenteron. The external layer of protoplasm forms the so-called blastoderm, and soon (Arnold, Lang) becomes thickened on the dorsal surface.
The embryo is next divided by two constrictions into three segments; and there are formed the three appendages corresponding to these, which are at first simple. The two posterior soon become biramous. The larva leaves the egg before any further appendages become formed.
Comparative development of the organs.
Central nervous system. The ventral nerve cord of the Crustacea develops as a thickening of the epiblast along the median ventral line; the differentiation of which commences in front, and thence extends backwards. The ventral cord is at first unsegmented. The supra-œsophageal ganglia originate as thickenings of the epiblast of the procephalic lobes.
The details of the above processes are still in most cases very imperfectly known. The fullest account we have is that of Reichenbach (No. [488]) for Astacus. He finds that the supra-œsophageal ganglia and ventral cord arise as a continuous formation, and not independently as would seem to be the case in Chætopoda. The supra-œsophageal ganglia are formed from the procephalic lobes. The first trace of them is visible in the form of a pair of pits, one on each side of the middle line. These pits become in the Nauplius stage very deep, and their walls are then continued into two ridges where the epiblast is several cells deep, which pass backwards one on each side of the mouth. The walls of the pits are believed by Reichenbach to give rise to the optic portions of the supra-œsophageal ganglia, and the epiblastic ridges to the remainder of the ganglia and to the circum-œsophageal commissures. At a much later stage, when the ambulatory feet have become formed, a median involution of epiblast in front of the mouth and between the two epiblast ridges gives rise to a central part of the supra-œsophageal ganglia. Five elements are thus believed by Reichenbach to be concerned in the formation of these ganglia, viz. two epiblast pits, two epiblast ridges, and an involution of epiblast between the latter. It should be noted however that the fate neither of the pair of pits, nor of the median involution, appears to have been satisfactorily worked out. The two epiblast ridges, which pass back from the supra-œsophageal ganglia on each side of the mouth, are continued as a pair of thickenings of the epiblast along the sides of a median ventral groove. This groove is deep in front and shallows out posteriorly. The thickenings on the sides of this groove no doubt give rise to the lateral halves of the ventral cord, and the cells of the groove itself are believed by Reichenbach, but it appears to me without sufficient evidence, to become invaginated also and to assist in forming the ventral cord. When the ventral cord becomes separated from the epiblast the two halves of it are united in the middle line, but it is markedly bilobed in section.
In the Isopoda it would appear both from Bobretzky’s and Bullar’s observations that the ventral nerve cord arises as an unpaired thickening of the epiblast in which there is no trace of anything like a median involution. After this thickening has become separated from the epiblast a slight median furrow indicates its constitution out of two lateral cords. The supra-œsophageal ganglia are stated to be developed quite simply as a pair of thickenings of the procephalic lobes, but whether they are from the first continuous with the ventral cord does not appear to have been determined.