298 PROFESSOR J. COSSAR EWART. 



(fig. 28, tr.). Cells of this type, with cells of the non-vascular somatic mesoderm of 

 the embryonic area, formed the true chorion (text-fig. L2, ch.), and they made up 

 the trophoblast forming the outer wall of the space containing the unsplit highly 

 vascular mesoderm which extended from the exoccelom to a short distance beyond 

 the sinus terminalis (fig. 34, s.t.). Typical columnar cells also occurred beyond the 

 sinus; they occupied, e.g., spaces between the trophoblastic discs (fig. 34, t.d.) and 

 between the shallow depressions scattered over the pointed end of the blastocyst. 



Though the simple columnar trophoblastic cells in the embryonic area are in con- 

 tact with the somatic mesoderm (fig. 34), and the corresponding cells beyond the 

 sinus terminalis are in contact with the yolk-sac endoderm (fig. 30), the cells forming 

 the trophoblast between the exocoelom and the sinus terminalis are not in contact 

 with the unsplit vascularised mesoderm. Evidence of this we have in fig. 28. 

 From this figure it is evident that there is a space between the trophoblast (tr.) and 

 the yolk-sac endoderm (y.e.), in which is suspended the rich plexus of vitelline vessels 

 carrying blood to and from the embryo. 



There is no evidence at the end of the third week that any of the cells of the 

 trophoblast up to the sinus terminalis are either phagocytic or in any way adapted 

 for fixing the blastocyst to the lining of the uterus. In all probability the simple 

 columnar trophoblastic cells simply imbibe the fluid portion of the uterine milk in 

 much the same way as the cells lining the intestine of the adult horse absorb the 

 fluid constituents of the food in the alimentary canal. This fluid on reaching the 

 space between the trophoblast and the yolk-sac endoderm has a chance of at once 

 entering the capillaries of the unsplit mesoderm and being conveyed to the embryo 

 by the vitelline veins. Taking into consideration the relatively great extent of the 

 vascularised mesoderm, it is extremely probable that the trophoblast extending 

 between the exocoelom and the sinus terminalis is the chief means by which nourish- 

 ment is secured for the embryo at the end of the third week. 



The second, or very tall, cells only occur in the fully developed trophoblastic 

 discs. These discs (text-fig. 2) are especially numerous near the edge of the vascu- 

 larised mesoderm which extends beyond the sinus terminalis (fig. 22 and text-fig. 4) — 

 in an area near the sinus measuring 10 by 3 mm. I counted 27 discs at various stages 

 of development. A disc 300 times enlarged is given in text-fig. 3, and a similar disc, 

 on a smaller scale, viewed from within, is given in text-fig. 4; sections of discs 

 are given in figs. 29 and 30. The majority of the sections of any given disc con- 

 tain so many nuclei (fig. 29a) that one is apt to assume that the peculiar tropho- 

 blastic projections from the 21 -days horse blastocyst consist of several layers of 

 cells, are in fact formed by a heaping up of cells, and are hence akin to the heaps 

 of cells met with in the trophoblast of Manis.* But sections carried through the 

 centre of a disc (fig. 29) clearly prove that even the largest discs consist of a single 



* Max Weber, " Beitriige zur Anatomie und Entwickelung der Genus Manis," Zoologische Ergebnisse einer Reise 

 in Niederliindisch Od-Indien, Leiden, 1892. 



