STUDIES IN MAMMALIAN EMBRYOLOGY. 521 



other processes of growth of these parts come under observatiou. 

 It is no longer in the lamellar fashion (of fig. 93) that the 

 cytoblastic elements come to be transformed into plasmodi- 

 blastic tissue, at least not generally. A phenomenon more 

 frequently observed in the later stages, when the uterine 

 swellings have reached the size of fig. 38 (cf. fig. 31) and 

 more, is represented under higher powers in figs. 90 — 92. 

 The separation between cytoblast and plasmodiblast is less 

 distinct. In the region immediately surrounding the villi 

 this was already noticed in comparing the two stages of figs. 81 

 and 82 that follow so closely upon each other. But whereas 

 in the stage of fig. 82 the distinction between cytoblast and 

 plasmodiblast had become difficult between the villi, fig. 9^! 

 (belonging to the same stage as fig. 82) shows that it was 

 yet very easy towards the lower surface of the placenta, where 

 the trophoblastic proliferation is most active. 



Later on, however, as rhe comparison of the last-named 

 figure (94) with those just nentioned (90 — 92) shows, the line 

 of demarcation becomes le^s marked. There is yet a certain 

 difference in the staining, and many of the cytoblast nuclei are 

 recognisable by their more copious absorption of picro-car- 

 minate. But one glance at the figs. 90 — 92 will convince us of 

 the very gradual transition between cytoblast and plasmo- 

 diblast. Also with respect to the fact that hardly any more 

 cell boundaries can be distinguished between the cytoblast 

 nuclei ; so that not only the plasmodiblast, but also the 

 cytoblast, should then be considered as a syncytium. 



Now the new process of transformation of cytoblastic into 

 plasmodiblastic — i. e. into sanguiniferous — tissue consists in 

 the appearance of nests of nuclei marked off by a comparatively 

 very distinct line of demarcation (fig. 91). These nests gra- 

 dually separate themselves from the surface, and are apparently 

 carried inwards (fig. 90), where for some time they persist 

 (fig. 92). After that they gradually develop cavities, into which 

 maternal blood-corpuscles are seen to penetrate, and which 

 thus become part of the general circulatory apparatus of the 

 plasmodiblastic syncytium. Fig. 91 clearly shows, however. 



