In Palæmon (Bobretzky) the primitive invagination ([fig. 239] A) has far smaller dimensions than in Astacus, and appears before the blastoderm cells have separated from the yolk pyramids. The cells which are situated at the bottom of it pass into the yolk, increase in number, and absorb the whole yolk, forming a solid mass of hypoblast in which the outlines of the individual cells would seem at first not to be distinct. The blastopore in the meantime becomes closed. Some of the nuclei now pass to the periphery of the yolk mass; the cells appertaining to them gradually become distinct and assume a pyramidal form ([fig. 239] B, hy), the inner ends of the cells losing themselves in a central mass of yolk, in the interior of which nuclei are at first present but soon disappear. The mesenteron thus becomes constituted of a layer of pyramidal cells which merge into a central mass of yolk. Some of the hypoblast cells adjoining the junction of the proctodæum and mesenteron become flattened, and in the neighbourhood of these cells a lumen first appears. The stomodæum and proctodæum are formed as in Astacus. [Fig. 239] B shews the relative positions of the proctodæum, stomodæum, and mesenteron. Although the process of formation of the hypoblast and mesenteron is essentially the same in Astacus and Palæmon, yet the differences between these two forms are very interesting, in that the yolk is external to the mesenteron in Astacus, but enclosed within it in Palæmon. This difference in the position of the yolk is rendered possible by the fact that the invaginated hypoblast cells in Palæmon do not, at first, form a continuous layer enclosing a central cavity, while they do so in Astacus.
Fig. 239. Two stages in the development of Palæmon seen in section.
(After Bobretzky.)
A. Gastrula stage.
B. Longitudinal section through a late stage. hy. hypoblast; sg. supra-œsophageal ganglion; vg. ventral nerve cord; hd. proctodæum; st. stomodæum.
The mesoblast appears to be formed of cells budded off from the anterior wall of the archenteron (Astacus, [fig. 237] D), or from its lateral walls generally (Palæmon). They make their first appearance soon after the invagination of the hypoblast has commenced. The mesoblast cells are at first spherical, and gradually spread, especially in an anterior direction, from their point of origin.
According to Reichenbach there are formed in Astacus at the Nauplius stage a number of peculiar cells which he speaks of as ‘secondary mesoblast cells.’ His account is not very clear or satisfactory, but it appears that they originate ([fig. 237] F) in the hypoblast cells by a kind of endogenous growth, and though they have at first certain peculiar characters they soon become indistinguishable from the remaining mesoblast cells.
Towards the end of the Nauplius period the secondary mesoblast cells aggregate themselves into a rod close to the epiblast in the median ventral line, and even bifurcate round the mouth and extend forwards to the extremity of the procephalic lobes. This rod of cells very soon vanishes, and the secondary mesoblast cells become indistinguishable from the primary. Reichenbach believes, on not very clear evidence, that these cells have to do with the formation of the blood.
General form of the body. The ventral thickening of epiblast or ventral plate, continuous with the invaginated patch already mentioned, forms the first indication of the embryo. It is at first oval, but soon becomes elongated and extended anteriorly into two lateral lobes—the procephalic lobes. Its bilateral symmetry is further indicated by a median longitudinal furrow. The posterior end of the ventral plate next becomes raised into a distinct lobe—the abdomen—which in Astacus at first lies in front of the still open blastopore. This lobe rapidly grows in size, and at its extremity is placed the narrow anal opening. It soon forms a well-marked abdomen bent forwards over the region in front ([figs. 239] B, and [240] A and B). Its early development as a distinct outgrowth causes it to be without yolk; and so to contrast very forcibly with the anterior thoracic and cephalic regions of the body. In most cases this process corresponds to the future abdomen, but in some cases (Loricata) it appears to include part of the thorax. Before it has reached a considerable development, three pairs of appendages spring from the region of the head, viz. two pairs of antennæ and the mandibles, and inaugurate a so-called Nauplius stage ([fig. 240] A). These three appendages are formed nearly simultaneously, but the hindermost appears to become visible slightly before the two others (Bobretzky). The mouth lies slightly behind the anterior pair of antennæ, but distinctly in front of the posterior pair. The other appendages, the number of which at the time of hatching varies greatly in the different Decapods (vide section on larval development), sprout in succession from before backwards ([fig. 240] B). The food-yolk in the head and thoracic region gradually becomes reduced in quantity with the growth of the embryo, and by the time of hatching the disparity in size between the thorax and abdomen has ceased to exist.
Isopoda. The early embryonic phases of the Isopoda have been studied by means of sections by Bobretzky (No. [498]) and Bullar (No. [499]) and have been found to present considerable variations. When laid the egg is enclosed in a chorion, but shortly after the commencement of segmentation (Ed. van Beneden and Bullar) a second membrane appears, which is probably of the nature of a larval membrane.