Embryonic Development in Man. 387 



smaller primitive streak of an earlier period has furnished these 

 masses (of mesoblast). Probably a small mass of mesoderm is 

 produced from a primitive streak at a very early stage, and later 

 this mass proliferates independently.' " ''Although this view of 

 Spec," Keibel continues, "appears to be the most probable, it is well 

 to point out that it is only an hypothesis, which encounters many 

 difficulties. We cannot at present state anything more definite as 

 to the origin of the mesoblast in man. Something more certain may 

 be said of the origin of the coelom, although its earliest stages 

 have also never been observed in man. It is certain, that, as in 

 mammals generally, the extraembryonic ca?lom (the cavity of the 

 ovum) is formed earlier than the embryonic ccrlom, and we are 

 probably correct if we assume that the extraembryonic coelom is 

 formed by cleavage (Spaltbildung) in a mesoblast which has already 

 developed. This would be in accordance with what is known of 

 the other mammals. 



The Trophohlast and Its SyncyUmn. 



The early characteristic tro])hoblast of the human ovum, though 

 first correctly described by Peters from his specimen, had pre- 

 viously been hypothetically constructed from the observation of an 

 older ovum ^?^ situ, in which the peculiar ectoblast shell had been 

 differentiated by the formation of the villi. We owe this description 

 to van Ileukelom^', who gives it in the following words: "One can 

 best get a conception of these conditions if one imagines all villi 

 connected at their periphery by beams of ectoblast, so that they 

 form an ectoblast shell full of small and large holes. This shell is 

 unevenly thick and rests directly on the maternal compacta. . . ." 



The trophoblast as found in our specimen surrounds the chorionic 

 cavity or cxocoelom of the ovum like a thick shell. It is, however, 

 not equally powerful on all sides, as has been already indicated 

 by the measurements of the trophoblast. Nor is it a solid mass of 

 cells. It is, on the contrary, honeycombed by irregular communi- 



"von Heukelom : Uelter die nienscliliche Placentatioii : Arcliiv f. Anat. u. 

 Pliysiologie (Abth. Anatomie), 1S9S. 



