POLYEMBRYONIC DEVELOPMENT IN TATUSIA 619 



appearance, and now show a decided increase in thickness (fig. 

 11). There is also a distinct tendency for some of the cell 

 walls to disappear, thus transforming Rauber's layer into a 

 syncytium. Furthermore, the surface of Rauber's layer presents 

 a 'fuzzy' appearance, the outer wall of some of the cells actually 

 being broken as though ruptures had resulted when the blasto- 

 cyst was freed from its insecure moorings to the mucosa. 



That all of these facts are evidence of the beginning of im- 

 plantation is clearly indicated by the act that the trophoblastic 

 cells which lie beyond Rauber's area are still unchanged, and 

 show the mosaic-like arrangement of polygonal cells so charac- 

 teristic of all of the free blastocysts. Unfortunately, there is 

 here a slight break in the series, so that we are not able to fol- 

 low up this clew through the obviously critical period of implan- 

 tation. We are therefore obliged to pass directly to an ac- 

 count of the modifications which are occurring both in the 

 mucosa epithelium and in the wall of the blastocyst in a vesicle 

 which has already become firmly anchored to the maternal 

 tissue. 



Blastocyst No. 316 represents the youngest firmly attached 

 stage that has been secured. A series of sections from this 

 specimen is shown in plate 2. In the living vesicle it could be 

 seen that, in addition to the small area which had established 

 an intimate union with the mucosa, almost the entire left side 

 was lying in contact with the uterine wall, and that as a result 

 the trophoblastic cells here had. greatly increased in thickness 

 (fig. 16). Whatever may be the nature of the stimulus which 

 causes the trophoblastic cells to react whenever brought into 

 close relation with the uterine epithelium, it is certain that the 

 influence- is not confined to Rauber's layer, but any portion of 

 the trophoblast, upon coming in contact with the mucosa, will 

 respond. It does not necessarily follow that such thickened 

 trophoblast establishes eventually a fusion or placental union 

 with the uterine wall, although a study of the stages more ad- 

 vanced than this one, suggests that some of it may so unite. 

 However, it should be pointed out that only in rare instances 

 does a blastocyst become attached in a manner such that an 



JOURNAL OF MORPHOLOGY, VOL. 24, NO. 4 



