26 



FOKMATION OF BLASTOCYST 



being somewhat oval, two nearly spherical. The observation shows that in the other Primates, 

 and therefore practically certainly also in man, segmentation takes place in the oviduct, and 

 after the same fashion as in the lower mammals. It will be observed that there is no zona 

 radiata represented. In most mammals the zona persists during the earlier phases of develop- 

 ment, and it is difficult to account for its absence in this and 

 other cases. It is doubtful whether it is to be ascribed to the 

 i^^_^^^ preservatives used, or to a normal precocious solution of the 



membrane. 



Fluid now appears between the peripheral layer 

 and the central mass, and separates them everywhere 

 except at one point (fig. 36, b). As the fluid accumu- 

 lates, the morula is converted into a vesicle (fig. 36, c), 

 the walls of which are formed of a single layer of small 

 clear cells, except at the point where the central mass 

 is attached and projects into the cavity (figs. 36, c, d, 



FIG. 35. SEGMENTING EGG OF and 37). The outer layer takes no part in the building 



up of the embryo, but is concerned solely with the 

 establishment of relations between the ovum and the 

 uterine mucosa. It has been termed by Hubrecht 



the trophoblast. The inner mass provides the material out of which embryo, 

 yolk-sac, and in man and apes almost certainly also the amnion, are formed, 



diameters. 



fern 



troph. 



fcm. 



troph. 



troph. 



fcm. 



FIG. 36. SECTIONS OF THE OVUM OF THE BABBIT 

 DUKING THE LATER STAGES OF SEGMENTATION, 

 SHOWING THE FORMATION OF THE BLASTOCYST. 

 (E. Van Beneden.) 



a, Section showing the enclosure of darker cells, 

 fcm., by clearer cells, tropli. : the enclosure is 

 usually complete ; &, more advanced stage in which 

 fluid is beginning to accumulate between the inner 

 and outer cells ; c, the fluid has much increased, 

 so that a large space separates inner from outer 

 cells except at one part ; d, blastocyst, its wall 

 formed of a layer of flattened cells (trophoblast), 

 with a patch of dark granular cells (formative 

 cell-mass) adhering to it at one part ; z.p., zona 

 pellucida. 



troph. 



and may be termed the formative or 

 embryonic cell-mass. At this stage the 

 ovum has usually been termed the 



blastodermic vesicle, but as the actual blastoderm is not yet formed it is 



better to call it the 



