424 IMPLANTATION OF THE OVUM IN 



(:01) further describes a porous condition in the zona and calls attention to the destruc- 

 tive action of many fixing fluids, which no doubt explains many discrepancies in the 

 various accounts heretofore given regarding the zona. All the specimens showing 

 zona in the accompanying plates were fixed by having been kept one day in Zenker's 

 fluid. At the stage immediately preceding fixation is found a blastocyst with an 

 outer layer of cells, the trophoblast of Hubrecht. These ectodermal cells are irregularly 

 rounded or cubical in shape; their nuclei are large and rounded and show numerous 

 mitoses. Figures 12 and 13 (PI. XXX) represent two sections from the same blas- 

 tocyst showing the trophoblast layer and within it the "inner cell-mass," whose cells 

 are beginning to be differentiated into formative ectoderm and entoderm. In Figure 

 5 there is shown another blastocyst, slightly older, but still prior to fixation, which 

 has the formative ectoderm on the right closely applied to the inner surface of the 

 trophoblast, while the entodermal cells are beginning to arrange themselves on the 

 opposite wall. In Figures 16, 17, 21, 22, 25, 30, and 32 interesting details in the 

 gradual differentiation of the entodermal layer may be noticed. The formative 

 ectoderm in the younger stages (PI. XXX, Figs. 17, 18, 21, 22; PI. XXXI, Fig. 25) 

 consists of a rounded clump of cells closely applied to each other and to the inner 

 side of the trophoblast layer, whose cells, it will be noticed, tend to become flattened 

 and here constitute what is known as Rauber's layer, or Deckschicht. In the older 

 stages (PL XXXI, Fig. 32) the formative ectoderm has by cell-multiplication grown 

 in all directions peripherally and has been brought to the free surface by the loss of 

 the cells of Rauber's layer. At the junction of the trophoblast with the borders of 

 the germinal area the differentiation is very distinct, both in the shape and structure 

 of the cells and their reaction to stains. In this respect there is a close resemblance 

 to similar stages in the development of tupaja as figured by Hubrecht ('95). Compare, 

 for example, Figures 21 and 32 in this paper with Hubrecht's Figures 54 and 68 respect- 

 ively. Very many interesting and important details in the differentiation of the 

 germ-layers of Sphermophilus must be left for consideration at another time, as many 

 of the sections which illustrate these points are not correspondingly good in regard 

 to the problem of fixation, and so are not figured here. 



2. Beginning of the Process of Fixation. A. UTERUS. Profound physiological 

 changes are inaugurated in the uterus with the impregnation of the female. The 

 uterine horns, which were small and pale in the resting condition, now become enlarged 

 and lengthened. The blood-vessels are congested, and there is a new growth of capil- 

 laries in the connective tissue of the mucosa. The connective-tissue cells themselves 

 rapidly divide and increase in volume, becoming rounded or oval with thickened 

 borders; they become closely massed together, giving a denser zone next the epithelium, 



