154 T. THOMSON FLYNN 



0-043 mm. opposite the vascular omphalopleure and 0-023 mm. 

 opposite the bilamiiiar. Similar proliferation and migration 

 of the nuclei have taken place but the Icjhular nests are much 

 smaller and less individual, and, further, more and more 

 nuclei remain outside the nests until in some places lobules 

 and nests as such are scarcely distinguishalde. The nuclei are 

 quite similar to the syncytial nuclei of tlie placental area being 

 rounded, bead-like, and vesicular with little chromatin. 



Chorion. — This shows, both in the amount of area of 

 attachment and of proliferation, a considerable advancement 

 on the condition found in the last stage. The fusion with 

 maternal tissue is so complete that the resulting layer is one 

 in which occasionally there is some difficulty in distinguishing 

 maternal and foetal cytoplasm. Here, as l)efore however, 

 we have the infallible test of the difference in structure, shape, 

 and fate of the maternal and foetal nuclei. 



The D i p 1 o p 1 a s m a . — The thickness of this layer measures 

 now from 0-135 to 0-190 mm. 



This increase is due mostly to the great growth of the 

 chorionic ectoderm, wdiich consists of a well-marked basal 

 cellular layer, the cytoblast, and a much thicker portion in 

 which cell outlines are not visible, the plasmodiblast. The 

 maternal portion of tlie diploplasma coiisists of the remains 

 of both the maternal syncytial protoplasm and of the syncytial 

 nuclei. When unaltered the latter have the characteristics 

 which have been before noted, viz. small rounded vesicles in 

 which the chromatin is suppressed but containing a more or 

 less prominent nucleolus (see tigs. 13. 14, and 19, syn.n.). 

 They have an exactly similar appearance to the nuclei of the 

 extra-placental area. 



At this stage, however, nests are seldom found intact. In 

 almost every case they have been invaded by plasmodiblast 

 nuclei {ph.n.), so that by this time a fair proportion of maternal 

 nuclei have degenerated and disappeared. 



The T r o p h o d e r m . — This is divisible as stated above into 

 cytoblast and plasmodiblast. 



The Cytoblast {ci/t.) is a remarkably definite laj-er of 



