DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 739 



VIII. General Development of the Trunk. 



After the closure of the blastopore, the definition of the embryo as a cylindrical 

 rod, pressing somewhat deeply into the surface of the yolk (PI. III. figs. 1-4), 

 becomes marked. Anteriorly its enlarged cephalic region soon rises boldly above 

 the surface of the periblast ; while the trunk, though prominently standing out 

 along the dorsum, and indenting the yolk in a pronounced manner ventrally, yet 

 laterally, by the alar expansion of the scutum on each side, gradually merges in the 

 general expanse of the blastodermic envelope, as observed in the serial sections 

 (figs. 3, 4, 5a-5d, PL IV.). The true limits of the embryonic trunk are in reality 

 not defined, the neurochord, ne, and myotomic masses, mes, are distinctly marked, but 

 more distally, in the snout and tail, as well as the lateral regions, no sharp line of 

 demarcation divides the young fish from the blastodermic area beyond. The embryonic 

 Selachian or bird is pinched off more or less sharply at an early stage ; but the Teleostean 

 embryo, instead of becoming folded off as it were from the j^olk, continues to lie 

 extended upon its surface, and gradually draws the vitellus into its large subenteric 

 enclosure, the abdominal walls, as we shall see ultimately, entirely encompassing the 

 yolk. In some forms the yolk persists less prominently than in others, the somato- 

 pleure more rapidly extending ventrally and enveloping it. It never projects, as in 

 Elasmobranchs and Sauropsida, in the form of a dilated sac distinctly separated from 

 the ventral surface of the body, except at one point, where a narrow vitelline stalk still 

 connects the two. Lereboullet speaks of such a pedicle in Esox (No. 95, p. 612); but 

 this has not been confirmed, and in no case probably does the splanchnopleure surround 

 the yolk, and form a narrow pedicel, until the latter has diminished to a very large extent. 



Ejpiblast. — The external epiblast undergoes little change. We have seen that it is 

 established as a single layer of cells, which very early become flattened and in section 

 spindle-shaped. They form, in fact, an epidermis or corneous stratum, ep; but are not for 

 some time marked off with any distinctness from the lower-layer cells of the blastoderm.* 

 In the region of the head they first show their characteristic features ; but they retain 

 their primary rounded, polygonal outline at the posterior extremity of the embryo till 

 much later (PI. IV. fig. 5c). These last-named cells, as remarked on a prior page, are 

 not differentiated fully from the cells beneath until the closure of the blastopore. While 

 over the trunk, and the area of the blastoderm beyond, the corneous epiblast extends as 

 a single stratum of squamous cells, yet it may often show slight proliferation, and 

 present more than one layer. In section, through the head of an early flounder 

 (PI. IV. fig. 3), this is so, though it is true the protrusion of the optic vesicles may 

 have cut off a thin superior stratum of neurodermal epiblast. Over the blastoderm 

 generally a single layer of corneous epiblast seems to be present ; the nervous layer, on 

 the contrary, is many-layered, and in the middle line becomes so dense as to form 



* In some Teleosteans this distinction would seem to be well marked, for Kowalewskt speaks of it as dis- 

 tinguishable soon after cleavage is ended in Carassius, Polyacanthus, and Gobius (op. cit, 1886). 



