FATE OF BLASTOPOKE, ETC., IN CHELOXIA. gy 



lips of the blastopore are fused in the median line, thus producing a 

 streak, and the yolk-phig reaches its final position, before the tail begins 

 to make itself perceptible (Woodcut X, C). This difference may be due 

 to the fact that in Ehismobranchii, the blastoderm has to enclose a lanjc 

 yolk-hoilfi before the lips of the blastoderm can unite in a streak, while 

 in Chelonia, only a semblance of the above process is gone through, 

 and the anterior lip of the blastopore has to encircle only a small 

 rudimentarij structure — the yolk-plug. Be this as it may, tVie chrono- 

 logical disparity will not appear serious in the eye of a comparative 

 embryologist, so long as the residts are alike, as is the case in this 

 instance. 



There is another fact which is of importance in comparing the 

 two gnjiips. In Elasmobranchii, when the blastoderm has grown 

 uniformly in all its circumference f )r some time, the caud;d swellings 

 begin to grow more rapidly than the adjacent portions of the edge 

 of the blastoderm, and couic to project heiiO)ul the hitter like a ln'nd of 

 tonfiuc orerliantjiiKj tlie jioW. (See the quotation above from SEDmvrcK, 

 '92, and his Figure 2 = Sedgwick, Fig. 2, PL YII of the present 

 article). In other words, the tail is formed in a manner somewhat 

 different from the rest of the eml)ryonic body and stands out freely as 

 a tongue. Xow, in describing the formation of the tail in Chelonia in 

 the foregoing pages. I liave re])eatedly shown that the tail in that 

 group is formed lii/ a iieir elerotioii or upheaval from the surface of 

 the blastoderm. The tail may be said to stand iip from the surf ice 

 of the blastoderm like a tongue. This can only be interpreted as 

 repeating tl»e process seen in the Elasmobranchii. The only difference 

 lies in the anachronism pointed out above, but this, as I have said, 

 can not be considered in comparing the two as a serious difficulty in 

 the way. 



Thus, whether we compare a stage of chelonian embryos, such as 



