336 RICHARD ASSHETON. 



the large segtneut will represent very well the more minute 

 structure of the embryo when in the two-segment stage, as in 

 figs. 1 and 2. 



Towards the periphery the protoplasm is very finely reticular, 

 whilst more centrally the meshes are much larger. It would 

 seem really to be vesicular, the vesicles being filled with oily 

 " yolk,^^ as mentioned above. 



The less vacuolated cortical layer makes inroads at two 

 points into the central portion. The nucleus is spherical and 

 sharply defined by a very deeply staining membrane. A plate 

 of very fine chromatin granules passes through its centre. 

 There are no oily vacuoles in the nucleus. 



In fig. 6 I give a drawing of a rather more advanced embryo. 

 In this there were about twelve to fourteen segments. There 

 was a very great disparity in the size of the several segments. 

 Such a stage may be easily derived from the foregoing condi- 

 tion of fig. 13. The large segment has divided, as well as 

 several of the smaller ones. This specimen, together with 

 the next one which I shall describe, and those of which figs. 

 1 and 2, 12 and 13 are drawings, were all obtained from the 

 same horn of a uterus. Their age was six days. The same 

 uterus contained embryos in which the cavity of the blasto- 

 dermic vesicle was established. 



Fig. 8 represents a specimen which presented a unique 

 stage. Segmentation had proceeded very much further than iu 

 the preceding case. At one point a single large segment 

 protruded beyond the other. Over the rest of the embryo 

 the outlines of the segments formed tolerably regular figures, 

 showing that the segments on the surface were approxi- 

 mately uniform in size and somewhat compressed by their 

 surroundings. 



This is the only specimen I have found in this condition. 

 It was unfortunately lost. 



This stage is succeeded by one which may be described as 

 a typical mammalian morula (fig. 7). In surface view the 

 segments present no great disparity of size. The embryo as a 

 whole is spherical. A series of sections shows that there is a 



