BAllLY STAGES OF DEVELOPMENT OF THE MOUSK. 81 



blastic nature of the trophoblast. In L the outer layer of cells are hypoblast, 

 the inner mass embryonic epiblast. lu N the hypoblast of the inner mass is 

 seen to have a common origin with the outer layer, since in M the blasto- 

 cystic cavity appears inside the hypoblast. In the hypoblast proper is 

 growing round inside the trophoblast. 



PLATE 6. 



Fig. 1. — A young blastocyst (95 cells) in which the cavity is small, and 

 the inner mass undififerentiated. 



Fig. 2. — A slightly older blastocyst (140 cells). The inner mass is still 

 undifferentiated. Owing to the slight obliquity of the section the cavity does 

 not appear to be as large as it should. 



Fig. 3. — A blastocyst of 170 nuclei, in which the inner mass is beginning 

 to differentiate into epiblast and hypoblast, and iu which the trophoblastic 

 cells which overlie the inner mass are slightly flattened. 



Fig. 4. — A blastocyst of only 105 cells, but in which the inner mass is 

 already differentiated. 



Fig. 5. — Transverse section through the embryonic end of a blastocyst of 

 about the same age as Fig. 3. The trophoblast cells are quite distinct from 

 those of the inner mass. 



Fig. 6. — A blastocyst of about 200 nuclei. The trophoblast is much 

 crumpled and folded, and is flattened over the inner mass. The latter is well 

 differentiated into a rounded epiblastic knob and scattered hypoblaslic cells. 



Fig. 7. — A much enlarged though still unfixed blastocyst with nearly 300 

 nuclei. Over the epiblastic mass the trophoblast is very much stretched, 

 and the hypoblastic cells are commencing to grow round the inside of the 

 blastocyst. 



Fig. 8. — A very much older embryo which is firmly embedded in the 

 uterine crypt, of which the epithelium has disappeared. The trophoblast has 

 proliferated to form the " Trager," which has invaginated the epiblastic knob, 

 iu which the amniotic cavity is apparent, into the yolk-sac. 



Figs. 1 — 7 were drawn with Zeiss, obj. 2 mm., comji. oc. 4, X 740; 

 Fig. 8 with Zeiss, obj. D, comp. oc. 4, x 350. 



Am. Amniotic cavity. E. Embryonic epiblast. H. Hypoblast. /. Inner 

 mass. T. Trophoblast. Tr. Trager. Y. S. Yolk-sac. 



VOL. 43, PART 1. NEW SEIilES. 



