84 



K. MI'ISUKURI; 



hnve hroiiu-lit ont, in tlie conv.se of the pi-esent ai'ticle, for the group of 



CheJoiiia. Tlie similarities are so striking and obvious that it seems 



hardly necessnrv to ])«)int them out. Let us take tlie Chdouia embryo 



represented in Figs. 8, <Sr/, and Series IX. There is the tail swelling 



enclosing within it an inditferent cellular mass in which all the layers 



are fused. The medidlary chord ends ])osteriorly in this mass. The 



medullary cau'd therefore ends just in front of this mass and is at this 



point connected with tlie enteron by the neurenteric canal into Avhich 



the posterior half of the origiiial ai'chenteron has been transformed 



(see Woodcut IV; also cf. Caiitnh. 1^). The hindmost end of 



the medullary canal or tlie neurenteric canal is still open dorsally, 



and from tins opening there arises a median streak which is marked 



for the most part by a groove lying on it, and along which the epiblast 



is proliferating cells to the indifferent cell-mass within. This streak 



runs backwards along the dorsal surface of the tail-swelling (Series 



IX, Sections f/-//), around aud down its ])Osterior tip-end to the ventral 



surface (Sections h-l), and tlien backwards along the median line of 



the non-embryonic epiblast below the tail-swelling (Sections k-iii), 



until it reaches the yolk-plug at its posterior end (Section //). The 



\i; J 4 viw streak diaurammaticallv represented is 



Woodcut XIV. ^^ ^ 



therefore as in the adjoined AYoodcut. 

 Comparing this with Woodcuts XII 

 or XIII, we can homologise the two, 

 ])(jint for point, and we can draw the 

 Yk. Pltig. t^vo very important conclusions (1) 

 that the aforesaid streak in the Clielo- 

 nia embryo, given in Figs. (S and (Sa 

 and Series IX, represents the blastopore in a condition observaljle in 

 Elasmobranch embryos of such a stage as is given in Woodcut XIII ; 

 and (2) that the problematical cell-mass (Series IX, //) wliich I 



