STUDIES IN MAMMALIAN EMBRYOLOGY. 559 



PLATE XXXIX. 



EiG, 42.— The uterus (Cat. n°' Sorex 73) from which the blastocysts of 

 this and the foregoing plate were taken. Natural size ; drawn from the 

 spirit specimen. 



Figs. 43 — 52. — Ten sections (Zeiss' apochr., oc. 4, obj. 4) through dif- 

 ferent regions of the embryonic shield of Fig. 34. The exact situation and 

 extent of the sections are marked in that figure by numbered arrows. 



The protochordal jjlate, pp., is cut in Figs. 43 and 44, and is giving origin 

 to mesoblast-cells. 



Mas. Utr. Cat. n"- Sorex 73 d, 2 n 19 s. 



„ 22 s. 



In Figs. 45 and 46 there is as yet no mesoblast laterally. The median 

 cells, pw., belong to the foremost extension of the protochordal wedge, 

 which is not yet confluent with the protochordal plate. A thickened 

 keel of epiblast is present in the median line. 



Mus. Utr. Cat. n"- Sorex 73 d, 2 r. 30 s. 

 „ „ „ 3 r. 1 s. 



In Fig. 47 the protochordal wedge, pw., is connected with lateral meso- 

 blast plates ; the mesoblast is also present in the annular zone of modi- 

 fied hypoblast, hi/, az. 



Mus. Utr. Cat. n°- Sorex 73 d, 3 r. 4 s. 



In Figs. 48 — 50 the unfolding of the lips of the paliugenetic blastopore 

 (front end of the gastrula-ridge) is especially distinct and expressed 

 even in the position of the cells. 



In Fig. 50 a faint trace of a partial perforation is observed in the median 

 line. 



Fig. 48.— Mus. Utr. Cat. u°' 73 d, 3 r. 6 s. 



Fig. 49.— „ „ „ 7 s. 



Fig. 50.— „ „ „ 8 s. 



Figs. 51 and 52 are sections further backwards through the gastrula- 

 ridge and its lateral plates of mesoblast. 

 Mus. Utr. Cat. n°- 73 d, 3 r. 15 s. 

 „ » » 2o s. 



Figs. 53 — 55. — Three sections through the whole extent of embryonic 

 shield and annular zone of modified hypoblast {hy, az.) of the blastocyst of 

 Fig. 33 (Zeiss' apochr., oc.4, obj. 8). pp. Protochordal plate. E. Epiblastic 

 shield, hy. Unmodified flattened hypoblast both inside and outside the ring 

 above mentioned, tr. Trophoblast. ^?<?. Protochordal wedge, yr. Gastrula-ridge. 



Fig. 53.— Mus. Utr. Cat. n°- Sorex 73 l-, 2 r. 20 s. 



Fig. 54.— „ „ „ „ 14 *. 



Fig. 55.— „ „ „ 1 r. 19 s. 



Fig. 56. — Section through the posterior knob of the gastrula-ridge with 



