68 J. F. HILL. 



diameter varyin,^ from 3-5 to 4 mm.) in all except two of the 

 blastocysts, in which it is elongate, with longer and shorter 

 diameters. It occupies about a third or less of the entire 

 extent of the vesicle wall, and thus appears relatively smaller 

 than that of the preceding (5, '01) vesicles. The entoderm now 

 extends for a distance of about 1 mm. beyond the limits of 

 the area, so that in the entire vesicle (fig. 40) three zones 

 differing in opacity are distinguishable, viz. the dense hemi- 

 spherical zone at the upper pole, constituted by the embryonal 

 area ; below that, a less dense, narrow annular zone, formed of 

 extra-embryonal ectoderm and the underlying peripheral 

 extension of the entoderm ; and finally, the still less dense 

 hemispherical area, forming the lower hemisphere of the 

 blastocyst and constituted solely by extra-embryonal ecto- 

 derm. Thus approximately the upper half of the blastocyst 

 is bilaminar, the lower half unilaminar. Sections show that 

 the embryonal ectoderm (fig. 79, emh. ect.) is now a quite 

 thick layer of approximately cubical cells, whilst the extra- 

 embryonal ectoderm {tr. ect.) is formed of relatively thin 

 flattened cells. The line of junction between the two is per- 

 fectlv obvious, both in sections (fig. 79) and in surface view 

 (PI. o, fig. 50). The embryonal ectodermal cells, though 

 much thicker than the extra-embryonal, are of less superficial 

 extent ; their nuclei therefore lie closer together than those 

 of the latter, moreover they are larger, stain more deeply, and 

 are more frequently found in division, all of Avhich facts 

 testify to the much greater growth-activity of the embryonal 

 as compared with the exti-a-embryonal ectoderm at this stage 

 of development (cf . fig. 50, emh. ect. and tr. ect.; in the prepara- 

 tion from which this micro-photograph was made the entoderm 

 underlying the embryonal ectoderm has been removed, whilst 

 it is still partially present over the extra-embryonal ectoderm). 

 The entoderm (fig. 79, ent.) over the region of the em- 

 bryonal area is readily separable as a quite thin membrane, 

 and is then seen to consist of squamous cells, polygonal in 

 outline, and either in direct apposition by their edges or con- 

 nected together by minute cytoplasmic processes. Beyond the 



