FATE OF BLASTOPORE, ETC., IN CHELOXIA. 77 



As the tail is formed l)y the rising of tlie anterif^r thicker half of 

 the primitive streak, the primitive streak and groove naturally cross 

 the posterior boundary (^f the tail at right angles (Wcjodciit X, C). 

 They should also of necessity be found over the dorsal median line 

 of the tail, and, when the latter attains some height, around its 

 tip on to the ventral median line, and then be ccnitiniied to the 

 thinner part of the primitive streak outside the emljryo proper until 

 they reach the yolk-phig. They are thus g-shaped. Such a condition 

 is actually seen in Chelonia up to quite a, late si age (Series IX). In 

 Clcmnn/s, the anterior part of the primitive streak and groove over the 

 tail-swelHng disappears rather early (Figs. 12, lo & Series X, XI). 

 In Trio)iiix, the primitive groove does not exist at all from the iirst, and 

 the streak also disappears quite early from the tail part (^Series A' III). 



In the tail which lengthens itself backwards, the formation of the 

 different parts of tlie embryo proceeds backwards, i.e., the medullary 

 chord, the not(3chord, the enteron, and the mesoblast are dilterentiated 

 in situ in the tissue surrounded by the epil^last. As this is derived from 

 the " Endwidst," we can not regard the latter solely as the mesoblast 

 but as something mi generis. 



The allantois is formed on the ventral surface of the " Endwulst" 

 mass : hence its stalk is continuous with that mass. 



The primitive streak finally disappears entirely by the separation 

 of the layers, with the exception of a- short stretch oi; the ventral 

 surface of the tail. The proctodeum is formed at tins point. 



The yolk-plug, at the latest stage observed, stands out as an ap- 

 pendage of the epiblast (Series XIII). Probably it persists to a late 

 stage as a rudimentary useless structure. In Cleiinitijs, it is elongated 

 posteriorly and forms on the fl<3or of the posterior aminotic tube, a pro- 

 minent ridge, the significance of which is not clear (Fig. 15 and 

 Series XI). 



