ELASMOBRANGHII. 



45 



t/i';P 



vitelline canal, connecting the yolk sack with the alimentary cavity. 

 The umbilical canal falls into the alimentary 

 tract immediately behind the entrance of the 

 hepatic duct. 



At a fairly early stage of development a 

 rod is constricted off from the dorsal wall of 

 the alimentary canal (figs. 27* and 23 x), which 

 is known as the subnotochordal rod. It is 

 placed immediately below the notochord, and 

 disappears during embryonic life. 



General features of the Elasmohranch embryo 

 at successive stages. 



Shortly after the three germinal layers be- 

 come definitely established, the rudiment of the 

 embryo, as visible from the surface, consists of 

 an oblong plate, which extends inwards from 

 the periphery of the blastoderm, and is bound- 

 ed on its inner side by a head-fold and two 

 lateral folds (fig. 28 B). This plate is the me- 

 dullary plate ; along its axial line is a shallow 

 groove — the medullary groove {mg). The ru- 

 diment of the embryo rapidly increases in 

 length, and takes a spatula-like form (fig. 28 C). 

 The front part of it, turned away from the edge 

 of the blastoderm, soon becomes dilated into a 

 broad plate, — the cephalic plate (h) — while the 

 tail end at the edge of the blastoderm is also 

 enlarged, being formed of a pair of swellings — 

 the tail swellings (ts) — derived from the lateral 

 parts of the original embryonic rim. By this 

 stage a certain number of mesoblastic somites 

 have become formed but are not shewn in my figure. They are the 

 foremost somites of the trunk, and those behind them continue to be 

 added, like the segments in Chaetopods, between the last formed somite 

 and the end of the body. The increase in length of the body mainly 

 takes place by growth in the region between the last mesoblastic 

 somite and the end of the tail. The anterior part of the body is 

 now completely folded off from the blastoderm, and the medullary 

 groove of the earlier stage has become converted into a closed 

 canal. 



By the next stage (fig. 28 D) the embryo has become so much 

 folded off from the yolk both in front and behind that the separate 

 parts of it begin to be easily recognizable. 



The embryo is attached to the yolk by a distinct stalk or cord, 

 which in the succeeding stages gradually narrows and elongates, and 

 is known as the umbilical cord (so. s.). The medullary canal has now 



Fig. 27*. Teans- 

 verse section through 

 the tail region of a 



PrISTIURUS EMBRYO OF 

 THE SAME AGE AS FIG. 28 

 E. 



df. dorsal fin; sp.c. 

 spinal cord; pp. body- 

 cavity; sp. splanchnic 

 layer of mesoblast; so. 

 somatic layer of meso- 

 blast ; mp. commencing 

 differentiation of mus- 

 cles; ch. notochord; x. 

 subnotochordal rod aris- 

 ing as an outgrowth of 

 the dorsal wall of the 

 alimentary tract; al. ali- 

 mentary tract. 



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