STUDIES TN MAMMALIAN EMBEYOLOGY. 555 



EXPLANATION OF PLATES XXXVI— XLII, 



Illustrating Prof. A. A. W. Hubrecht's paper " Studies in 

 Mammalian Embryology. II. — The Development of the 

 Germinal Layers of Sorex vulgaris." 



PLATE XXXVI. 



Figs. 1 — 4. — (The numbers in brackets refer to the catalogue number of 

 the uterus from which the embryos were taken.) Four uteri of Sorex con- 

 taining the earliest embryonic stages that were investigated. The swellings 

 visible in 16 are not caused by the enclosed embryos, but are the last visible 

 remnants of a preceding pregnancy, and mark the arese of placentation in 

 their latest retrogressive phase. 



These figures are given in natural size. The unpaired median portion is 

 the vaginal part in which embryos are never found. 



Figs. 5 — 7. — Transverse sections of three young blastocysts of No. 16. 

 Z. Zona pellucida. Tr. Trophoblast. £". The inner cell-mass which will 

 give rise to epiblast and hypoblast of the embryo. 



In Fig. 5 there is a difference in size between certain cells of the inner 

 mass, which was not noticed in Figs. 6 and 7. In all there is sufficient 

 evidence of trophoblast-cells between the inner mass and the zona 

 pellucida. The total number of cells constituting these blastocysts 

 varies from fifty to sixty. The figures were drawn (as are all the 

 other figures of sections on this plate) with Zeiss' apochr., oc. 4, obj. 4. 

 Fig. 5.— Mus. Utr. Cat. n"- Sorex 16 a and 6, 3 r. ^s> 

 Fig. 6.— „ „ 16 eg, 2 r. 14 s. 



Fig. 7.— „ „ 16 Off, 2 r. 14 s. 



Fig. 8. — Section through an embryo of No. 44. Lettering as in the pre- 

 ceding figures. There is an artificial bulging in on one side of the blastocyst, 

 which was absent in other embryos of the same age, and which must un- 

 doubtedly be ascribed to the thinning out of the zona simultaneously with the 

 increase in size of the blastocyst. It must be ascribed to the reagents used 

 in hardening the tissues. 



It will be seen from the subsequent figures on this and the following plates 

 that this phenomenon is very much on the increase as the size still further 

 augments, and is only counteracted when once the blastocyst has become 

 attached to the walls of the uterus (see Fig. 12). 

 Mus. Utr. Cat. n"- Sorex 44 a, 5, 4 r. 11 s. 



1 See this Journal, vol. xxx, p. 393. 



