STUDIES IN MAMMALIAN EMBRYOLOGY. 503 



known and published facts, for which we are indebted to those 

 different authors, and professes to be an attempt (based on 

 independent observations) towards harmonious interpretation, 

 rather than a contribution towards polemics and criticism, I 

 am all the more inclined to refrain from further '' weighinar 

 of evidence." 



A.nd so now a full description of the chief points which I 

 have noticed in the early developmental stages of the shrew 

 may here follow. 



Chap, I. — Early Developmental Stages of 

 SoREX Vulgaris. 



1. The Blastula and the Didermic Blastocyst. 



The earliest ovum of which I possess transverse sections is 

 in a stage just following upon the phase of the well-known 

 rabbit's ovum, which was described and figured by van 

 Beneden, and has since passed into every text-book of embryo- 

 logy as a specimen of the earliest mammalian blastocyst. 



There is a very distinct zona pellucida, internally clothed by 

 a layer of flattened cells, of which I count six to eight in the 

 largest circumference, and at one spot there is an agglomera- 

 tion of larger and more bulky cells of somewhat different size. 

 The first-named layer is the trophoblast (vide Hubrecht, '' The 

 Placentation of Erinaceus,^' ^ Quart. Journ. of Micr. Sci.,^ vol. 

 XXX, p. 298). The accumulation of cells contains the material 

 for the embryonic epiblast and for the hypoblast (figs. 5 — 7). 

 Counting the nuclei in this and the remaining sections, the 

 embryo here figured is found to consist of from fifty to sixty cells. 

 A cavity which is clearly marked though not yet over-spacious 

 extends beneath the polar thickening. It is, properly speak- 

 ing, the segmentation cavity ; after the development of the 

 coenogenetic hypoblast the greater part of it is enclosed bv 

 hypoblast- cells, and becomes the cavity of the yolk-sac. The 

 trophoblast forms its outer wall, the thick zona is agam out- 

 side this. The embryos here described were found freely 

 suspended in the lumen of the uterus, without any attachment 



