740 Bashford Dean Memorial Volume 



mately the same stage, it has an unusually large yolk mass, probably about 50 mm. in 

 diameter. The length of the embryo (when straightened) equals about one-fourth of 

 the diameter of the yolk mass. One wishes for similar data concerning modern sharks, 

 but a cursory search of the literattire reveals nothing that is helpful in this connection. 



LATER EMBRYONIC DEVELOPMENT 



The account ot the embryonic development of Heterodontus japonicus has been 

 interrupted quite arbitrarily, following the stage with four pairs of mesoblastic somites, 

 in order to consider the yolk blastopore before reaching a stage too far removed from 

 its origin. The remaining stages of embrv^onic development, as represented in Dean's 

 drawings, unll now be considered in serial order. 



Figure 17, plate I, represents a cleared embryo with. 12 pairs of complete and one 

 pair of posterior incomplete somites. The course of the right neural fold in this figure is 

 not easy to follow, but it seems quite certain that the neural folds have almost met 

 anteriorly as well as posteriorly. What appears to be a neural fold on the right side of the 

 figure is the floor of the neural plate seen in optical section. According to this interpreta- 

 tion, the head region has rotated anticlockwise through almost 90 degrees. In the absence 

 of rotation, it would be impossible to obtain a side view of the beginning cephalic flexure 

 and the structures underlying the brain. The pocket situated ventrad and caudad 

 to the brain is the fore-gut. The arch-Hke anterior intestinal portal, leading from the 

 broad subgerminal cavity into the narrower fore-gut, is plainly visible. An embryo of 

 Squalus acanthias possessing the same number of somites has the neural tube almost closed 

 throughout its length. 



Figure IS, plate I, represents a cleared embryo with 12 pairs of complete somites, 

 one posterior and two anterior incomplete somites. The latter appear to be undergoing 

 degeneration. The neural folds appear to be united in their middle thirds. Anteriorly, 

 the folds appear less close together than in Figure 17, and the cephalic flexure is less 

 pronounced: the amount of rotation in the head region is less. Evidently the brain is not 

 quite so far along in its development, despite the fact that some other structures are 

 slightly more advanced. The fore-gut and the anterior intestinal portal are well shov,Ti. 

 A sheet of mesoderm is found dorsal, anterior and ventral to the fore-gut; its ventral 

 portion is e\^dently mesenchymatous. The deep dent at the anterior end of the right 

 neural fold is an artifact. 



Figure 19, plate I, represents a cleared embryo with 14 pairs of complete somites and 

 one pair of anterior incomplete somites. The neural tube is closed except for a short 

 distance at each end. There is a pronounced cephaHc flexure and a beginning cervical 

 flexure. The fore-gut is enlarged dorso-ventrally. This drawing shows a decided under- 

 cutting and upHfting of both head and tail-bud. 



Figure 20, plate II, represents a cleared embryo with 18 complete somites and one 

 anterior incomplete somite. The right side only is shown, but presumably the left side 

 has the same number. The cephaKc and cervical flexures are sHghtly more pronounced 



