300 PROFESSOR J. COSSAR EWART. 



layer of long, narrow, columnar cells ; that proliferation, instead of leading to a 

 heaping up of cells, has simply increased the extent of the original single layer, 

 with the result that it has bulged outwards to form a dome-like projection with a 

 relatively small central cavity. 



The long cells forming the greater part of the disc, i.e. the part in contact with 

 the lining of the uterus, never seem to end in sac-like processes, but such processes 

 are usually seen protruding from some of the tall cells forming the edge of the discs 

 (fig. 29). It might hence be inferred that the discs are only, or at least mainly, con- 

 cerned with fixing the embryonic sac to the lining of the uterus. If, however, 

 sections of a disc are carefully examined, granules, and vacuoles containing one or 

 more deeply stained bodies, are invariably seen in or between the tall cells (fig. 29) ; 

 hence it may be assumed that the discs subserve nutrition as well as fixation.* 



As it happens, all the stages in the development of the trophoblastic discs are met 

 with at the end of the third week. Nothing of the nature of a basement membrane 

 or coagulum is present within the layer of simple columnar cells forming the tropho- 

 blast up to the sinus terminalis, but in sections of the 50-mm. blastocyst beyond the 

 unsplit mesoderm the cells of the trophoblast seem to rest on a basement membrane. 

 As offshoots from this apparent basement membrane extend up between the tropho- 

 blastic cells, and as the layer reaches a considerable thickness under the developing 

 and completed discs (figs. 29 and 30), it is evidently formed out of material taken in 

 by the epiblastic cells forming the trophoblast beyond the sinus terminalis. Under the 

 large disc represented in fig. 29 the coagulated material reaches a considerable thick- 

 ness, but under the developing disc given in fig. 30 the amount of the coagulum is 

 still limited. Sometimes a band connects the coagulum of one disc with that of an 

 adjacent disc. Whether the thickening of the coagulum at certain definite points is 

 due at the outset to increased activity of the cells under which it lies, or to a zone of 

 phagocytic cells around those destined to form the disc, I am unable to say. It is, 

 however, certain that as the coagulum increases in amount the cells lying over it 

 increase in number, and especially in length. 



It is well to bear in mind that these trophoblastic discs are only present for a 

 short time — they are not yet developed at the middle of the third week, and they 

 disappear before the end of the fourth week. 



The third kind of trophoblastic cells, instead of ending in square-cut ends, 

 terminate in sac-like processes separated from each other by more or less distinct 

 spaces. Cells of this type are always found in the grooves surrounding the discs 

 (fig. 30, pc.) ; they also occur in patches between the discs and line the shallow de- 

 pressions scattered over the distal end of the blastocyst. There is no evidence that 

 the sac-like projections of these cells either adhere to, fit in between, or destroy the 



* From the appearance of some of the round bodies seen in the disc represented in fig. 29 it is extremely 

 probable that had the 21-days blastocyst been fixed with osmic acid, fatty globules like those found by Jenkinson 

 in the sheep would have been met with in the cells forming the trophoblastic discs. Jenkinson, " Notes on the 

 Histology and Physiology of the Placenta in Ungulata," Proc. Zool. Soc, 1906. 



