DEVELOPMENT OF THE PIC4 DURING FIRST TEN DAYS. 347 



Weysse will probably stand alone in instituting a com- 

 parison with Araphioxus, but as regards the actual facts I am 

 able to support him to a certain extent. 



It is true that what I have found is in detail extremely 

 unlike the majority of Weysse's specimens. Yet my own 

 specimens differ so greatly one from another that it is probable 

 that almost any form may be assumed by the cell masses in 

 question. 



The course of events would seem to be as follows. During 

 the ninth day the epiblastic knob is a compact mass of cells, 

 nearly as thick as it is broad. The inner surface is characterised 

 by the presence of many small nuclei, while towards the surface 

 the nuclei are larger (fig. 29). (Compare Hubrecht [17], figs. 

 57, 59.) These features probably indicate a more rapid 

 growth of the inner parts of the epiblastic knob. The result 

 is that the knob acquires a convex surface on its inner face 

 (figs. 32, 33). An exaggeration of this process might no 

 doubt lead to a doubling up as in Tupaia or Talpa. Some- 

 thing of the kind does occur in the pig, but I do not 

 think it is so evident a phenomenon as it is in those animals. 

 Figs. 30 — 33 are drawings of four secfions through one 

 specimen. There are thirteen sections in all; fig. 30 is the 

 third. In this there is a distinct doubling up, and the 

 trophoblast is broken. In fig. 31, which is the fifth section, 

 the epiblastic knob is curved and bent away from the over- 

 lying trophoblast, which, however, is not broken. Figs. 32 

 and 33 are the ninth and tenth sections, and are therefore 

 near the other end of the series. These show that this part 

 of the epiblastic knob is not doubled up, but forms a solid 

 plate, which is not actually fused with the trophoblast above 

 it. The trophoblast overlying the epiblast is broken in 

 several places. 



Fig. 35, which represents a section of another specimen 

 from the same uterus, shows a very slight curvature of the 

 epiblastic plate. The overlying trophoblast is broken, and 

 pieces of it (T.R.) are to be seen lying upon the epiblast. In 

 another specimen (fig. 34) there is no curvature at all. The 



