A is a side view in optical section.
B and C are two later stages from the ventral (oral) surface.
ae. archenteron; sc. segmentation cavity; hy. hypoblast; me. mesoblast; ep. epiblast; m. mouth; st. stomach; pr.d. prostomial disc; po.d. metastomial disc; pr. proboscis.
Fig. 92. Three Stages in the development of Lineus.
(After Barrois.)
A. Side view of an embryo at a very early stage as an opaque object.
B and C. Two late stages, seen as transparent objects from the ventral surface.
ae. archenteron; m. mouth; pr.d. prostomial disc; po.d. metastomial disc; cs. lateral pit developing in B as a diverticulum from the œsophagus; pr. proboscis; ms. muscular layer (?); ls. larval skin about to be thrown off; me. mesoblast; st. stomach.
Immediately after the completion of the gastrula a remarkable series of phenomena takes place. The embryo when viewed from the ventral surface assumes a pentagonal form ([fig. 91] B), and four invaginations of the epiblast make their appearance on the ventral surface ([fig. 92] A), two in front of (pr.d.) and two behind (po.d.) the mouth; they result in the formation of four thickened discs. These discs soon become separated from the external skin, which closes in forming an unbroken layer over them ([fig. 91] C). The discs grow rapidly, and first the prostomial pair and subsequently the metastomial fuse together, and finally the whole four unite into a continuous ventral plate; analogous it would seem to the ventral plate of chætopodan and arthropodan embryos. The plate so formed gradually extends itself so as to close over the dorsal surface, and to form a complete skin within the primitive larval skin, which at this period is richly ciliated, though the embryo is not yet hatched ([fig. 91] C). While these changes are taking place, there are budded off from the invaginated discs a number of fatty cells, which fill up the space between the discs and the archenteron, and eventually form the mesoblastic reticulum. During this stage the rudiment of the proboscis also makes its appearance as a solid process of epiblast, which grows backwards from the point of fusion of the two prostomial discs at the front end of the embryo ([fig. 91] C, pr.). A lumen is excavated in it at a later period. The lateral organs or cephalic pits arise in a somewhat unexpected fashion as a pair of diverticula from the œsophagus ([fig. 92] B, cs.)[92], which soon fuse with the walls of the body at the junction of the prostomial and metastomial plates ([fig. 92] C, cs.), although they remain for some time attached to the œsophagus by a solid cord.
During these changes the original larval skin separates itself from the subjacent layer formed by the discs ([fig. 92], B and C), and is soon thrown off completely, leaving the already ciliated ([fig. 92] C) external layer of the invaginated discs as the external skin of the young Nemertine. During, and subsequently to, the casting off of the embryonic skin, important changes take place in the constitution of the various layers of the body, resulting in the formation of the vascular system and other mesoblastic organs, the nervous system, and the permanent alimentary tract. These changes appear to me to stand in need of further elucidation; and the account below must be received with a certain amount of caution.
It has been already stated that the two discs give rise to fatty cells, which occupy the space between the walls of the body and the archenteron. At the period of the casting off of the embryonic skin fresh changes take place. The discs become very much thickened, and then divide into two layers, which become the epidermis and subjacent muscular layers. The muscular layers arise in two masses, separated by the two cephalic sacks. The anterior mass is formed as an unpaired anterior thickening, followed by two lateral thickenings. The posterior mass is much thinner, in correspondence with the rapid elongation of the metastomial portion of the embryo.