POLYEMBRYONIC DEVELOPMENT IN TATUSIA 607 



evidence that the underlying entoderm takes part in this pro- 

 liferation of the mesodermal cells, but directly anterior to the 

 primitive streak the entoderm is found to be actively dividing to 

 form a group of cells, which can be traced throughout successive 

 stages to mesodermal tissue. Undoubtedly this center of pro- 

 liferation corresponds to the protochordal plate of Hubrecht 

 (fig. 25, P. PL). 



The conditions which we have here described for the pair of 

 Embryos I and II also hold for the other pair, or Embryos III 

 and IV. The relation of the various parts is identical in the two 

 pairs. The only point in which they do differ is the fact that 

 the two secondary buds III and IV are more widely separated 

 than I and II, and hence the ectodermal vesicle appears much 

 wider (cf. fig. 62 with fig. 64). 



Following the sections through from the anterior limits of 

 either pair of embryos towards the center of the vesicle, two 

 important changes are noticeable: First, the entoderm becomes 

 entirely separated and distinct from the ectoderm; that is, these 

 two layers have never become fused; and second, the roof of the 

 ectodermal vesicle thins out to a single layer, while to either side 

 its wall remains from two to three cells thick (fig. 63). These 

 lateral thickenings are found prior to the appearance of the em- 

 bryonic "rudiments, when the roof of the vesicle undergoes the 

 general reduction in thickness. 



Toward the posterior ends of the embryos the posterior 

 grooves fade out, completely disappearing before the sections 

 which cut the tip of the primary buds are reached (fig. 26). In 

 the case of Embryos III and IV the posterior ends of the em- 

 bryos extend well back into the primary buds, which are seen 

 to be sharply separated from each other, especially in the last 

 three or four sections which cut the ectodermal vesicle (fig. 26). 



The embryonic entoderm or gut-en toderm of each embryo is dif- 

 ferentiated from the primary 3'olk-sac entoderm. Apparently any 

 region of the yolk-sac where the embryonic buds happen to im- 

 pinge against its inner surface will differentiate into gut-entoderm. 



The conditions which we have just recorded may be further 

 chronicled by a brief account of specimen No. 175, which is the 



