ELASMOBRANCHII. 55 



General features of tlie Elasnwbranch embryo at successive stages. 



Shortly after the three germinal layers become 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 bounded on its inner 

 side by a head-fold and two lateral folds (fig. 28 B). This plate 

 is the medullary plate ; along its axial line is a shallow groove 

 the medullary groove (ing). The rudiment 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 (//) while the tail end at the edge of the blasto- 

 derm 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 become completely closed. The anterior 

 region constitutes the brain ; and in this part slight constrictions, 

 not perceptible in views of the embryo as a transparent object, 

 mark off three vesicles. These vesicles are known as the fore, 

 mid, and hind brain. From the fore-brain there is an outgrowth 

 on each side, the first rudiment of the optic vesicles {op). The 

 tail swellings are still conspicuous. 



