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 
ipa 25) 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 te 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-entoderm of each embryo is dif- 
ferentiated from the primary yolk-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 
