STUDIES IN MAMMALIAN EMBRYOLOGY. 657 



Fig. 22. — Trausverse section of a blastocyst (cf. Fig. 16) in which the 

 hypoblast is just being formed by cells detaching themselves from the polar 

 thickening (cf. Figs. 10 and 11), which has become more flattened. 



The zona is much thinner and more considerably folded. 



Lettering and magnifying scale as in Figs. 5 — 11. 

 Mus. Utr. Cat. n°- Sorex 122, 2, 1 r. 11 s. 



Fig. 23. — Part of the folded zona with trophoblast- and hypoblast-cells of a 

 similar embryo. The hypoblastic nuclei, hy.^ are distinctly larger sized than 

 those of the trophoblast, tr. 



Mus. Utr. Cat. n"- Sorex 122, 2, 1 r. 17 s. 



Fig. 24. — The embryonic shield of another blastocyst in the same stage. 

 Distinct flattened hypoblast-cells are present below the epiblastic shield, jB". 

 Mus. Utr. Cat. n"- Sorex 122, 2 n 7 5. 



Fig. 25. — Blastocyst of a later stage, having become didermic in its whole 

 extent. At the spots tr" . the trophoblast is thickened preliminary to the 

 attachment of the blastocyst to the uterine mucosa (Zeiss' apochr., oc. 4, 

 obj. 16). 



Mus. Utr. Cat. n«- Sorex 2<», 1 r. 30 s. 



Fig. 26. — The epiblastic shield of the same embryo (cf. Fig. 17) more 

 considerably enlarged (Zeiss' apochr., obj. 4, oc. 4). The zona, z., has become 

 extremely attenuated, tr' . Trophoblast-cells above the epiblastic embryonic 

 shield (cf. Figs. 5 and 7). In later phases these have entirely disappeared. 

 Mus. Utr. Cat. n»- Sorex 2'», 1 r. 29 s. 



Fig. 27. — The modified trophoblastic zone, tr" ., of Fig. 25, more consider- 

 ably enlarged (Zeiss' apochr., oc. 4, obj. 4). 

 Mus. Utr. Cat. n"- Sorex 2«, 1 r. 30 s. 



Figs. 28 and 29. — Two sections through another didermic blastocyst, of 

 which the former passes through the hypoblastic protochordal plate, fp, (cf. 

 Fig. 18). 



Mus. Utr. Cat. n°' Sorex 52 i, 4 r. 11 s. 

 „ „ 4 r. 3 s. 



Figs. 30 and 31. — Two sections through yet another didermic blastocyst 

 (cf. Fig. 19). The protochordal plate is cut along its longest diameter. 

 Mus. Utr. Cat. n"- Sorex 52 e, 3 /•. 26 s. 

 4 r. 8 s. 



