EMBRYOLOGY 409 



When the follicle is fully ripe in most mammals it ruptures spon- 

 taneously at a time that is associated with a general physiological 

 condition known as heat or oestrus. In the rabbit, as noted, the ripe 

 follicle does not burst until after copulation, which occurs during 

 oestrus. The ovum thus released is received by the oviducal funnel, 

 and at the top of the oviduct it is fertilised by the entrance of a single 

 sperm. This is followed by the formation and extrusion of the 

 second polar body. 



The egg when it leaves the follicle is a minute spherical cell 

 composed of semi-transparent protoplasm, and it contains a vesicular 

 nucleus with little chromatin but a well-marked karyosome. A few 

 yolk spheres may be present, but never many. It is surrounded 

 by an egg membrane, perhaps a secondary egg membrane, which 

 is termed the zona pellucida. This is frequently striated for part 

 or the whole of its thickness, suggesting that it is perforated by 

 minute canals, and so this -region is sometimes called the zona 

 radiata. 



The first cleavage is complete cutting the ovum into two 

 equal or approximately equal cells. The second cleavage is also 

 complete and generally simultaneous in the first two cells, and the 

 four resultant daughter cells are usually arranged in a characteristic 

 cross. After this the segmentation is more or less irregular, and as 

 a result of it there is produced a small solid mass of cells, the morula. 

 At quite an early stage a certain amount of organisation can be 

 distinguished in this morula, for it will be seen that the outer cells 

 form a definite peripheral layer around a more irregular internal 

 group. As these changes occur while the egg is still surrounded by 

 the zona, the superficial layer is termed the sub-zonal layer and 

 the remainder the internal cell mass. As segmentation continues a 

 cleft appears between the two groups of cells, and it enlarges until 

 the internal cell mass is separated from the sub-zona layer by a 

 fluid- filled cavity, save at one point where they remain in contact. 

 Thus we have a hollow sphere with a wall one cell thick, the blasto- 

 dermic vesicle, supporting within it at its upper pole the inner cell 

 mass. Its cavity is taken to represent the yolk mass of an egg, like 

 that of the fowl. That is to say, the cavity is the subgerminal 

 cavity extended to include the whole of a hypothetical yolk mass, 

 which is replaced by a small amount of fluid. The vesicle wall is 

 regarded as ectodermal, but since it takes no part in the actual 

 formation of the embryo it must be regarded as corresponding 

 to the extra-embryonal ectoderm of the chick. By the time this 

 stage has been reached the egg has traversed the Fallopian tube 

 and lies in the uterine cavity, and since the vesicle wall is the part 

 brought into contact with the uterus it is obvious that it will be 



