Embryonic Development in Man. 389 



corpuscles, and the whole cells are three to four times as large 

 in diameter as an erythrocyte. Karyokinetic figures are occasion- 

 ally, though not very frequently, seen. (Fig. 29.) 



The trophohlast cells next to the chorionic cavity are cuboidal 

 and compact. Their protoplasm is rather scanty, and it stains much 

 deeper with eosin than that of the cells of the middle tropho- 

 hlast stratum; the nuclei are somewhat smaller, more oval and 

 slightly richer in chromatin. Towards the uterine cavity the 

 trophohlast cells form thin bands which in sections present them- 

 selves as slender bridges connecting the trophohlast with the struct- 

 ure mentioned before as ''the thin strip of the decidua capsularis." 

 Where they lead up to the surface the trophohlast cells have often 

 broken through the syncytium. Here the cells become fusiform, 

 bipolar and while not showing any marked features of degeneration, 

 it becomes difficult to distinguish them from what appear to be 

 decidual cells. It has previously been stated that the decidua cap- 

 sularis separating the outer pole of the ovum from the uterine 

 cavity is deficient in some portions, so that the ovum, in fact, is not 

 yet entirely separated from the uterine cavity. This impression 

 is very strongly conveyed by one of the sections which for some 

 reason was cut much thinner than the others (it is certainly less 

 than 5 micra). Here one can see that the trophohlast cells have 

 proliferated outwardly, have broken thi-ough the syncytium and 

 have become fusiform. They reach to and form the very surface. 

 In this section decidual cells appear to be absent from the strip 

 which borders upon the cavity of the uterus. However, even in 

 this strip some undoubtedly maternarcell elements can be recognized, 

 namely, mononuclear lymphocytes and polynuclear leucocytes, and 

 also, of course, infiltrating erythrocytes. 



The trophohlast cells, whether they be present in a single layer, as 

 towards the chorionic cavity, or whether they form larger or smaller 

 irregular masses honeycombed by blood spaces, are covered by the 

 syncytium. This consists of a layer of protoplasm in which cell 

 boundaries are not demonstrable. In sections the protoplasmic layer 

 is generally rather narrow, but there and there it is thickened, form- 

 ing projections. These are seen particularly in the most peripheral 



