THE vertebrates: the amphibia and birds 155 



the entry of a second, in highly yolky eggs such as birds' this mechanism 

 breaks down, and a number of sperm penetrate. Only one of these com- 

 pletes fertilisation by fusing with the egg nucleus; the remainder form 

 subsidiary nuclei which probably play a part in digesting the yolk in the 

 very early stages of development. In the next events, those of cleavage, 

 the influence of the yolk is even greater. The cleavage furrows start in 

 the clear cytoplasmic region, and never succeed in forcing their way down 

 into the inert mass of yolk. The first two cleavage planes are vertical, but 

 the third, while also starting as a vertical furrow in the flat layer of cyto- 

 plasm, soon curves round so as to run horizontally parallel to the surface, 

 and thus cuts underneath the cytoplasm and separates it from the yolk. 

 The cleavage pattern, even in these very early stages, is irregular, and we 

 have httle exact knowledge of how the cleavage planes are related to 

 later development. 



The cleavages convert the cytoplasmic region into a small compact 

 plate several cells thick. The cells contain fairly large quantities of yolk 

 granules, and at the edges the plate merges into the uncleaved yolk 

 through a zone of increasingly large and more yolky cells. Beneath the 

 mass, however, the yolk begins to liquify, and in sections this region 

 appears as an empty space, the 'subgerminal cavity'. There is considerable 

 debate as to exactly what happens next; and on this turns the question 

 of whether the subgerminal cavity is considered to be equivalent to the 

 blastocoel cavity of the amphibian egg or as a mere local modification 

 of the yolk. The thin plate of cells, lying on the massive yolk, is very 

 easily shrunken and distorted by normal histological methods of fixation 

 and presents great difficulties to the experimentalist; it will not be until 

 ways are found of overcoming these that we shall reach a fully satisfactory 

 interpretation of events. At present, one of the main views (held by Pasteels 

 1936-7, Peter 1938 and others) is that the subgerminal cavity is not 

 equivalent to the blastocoel, but that the latter soon begins to appear in 

 the form of irregular horizontal sphts within the mass of cells; these 

 cavities gradually expand and run together until they form a thin space 

 separating an upper from a lower layer. This space would then be the 

 blastocoel, and the lower layer of cells would correspond with the large 

 yolk-laden cells at the vegetative end of an amphibian blastula. In the 

 bird, these authors hold, the lower layer merely stays where it is during 

 gastrulation and forms the endoderm (Fig 4.1, p. 59). 



Others believe that the endoderm, instead of merely splitting off from 

 the upper layer, is derived from it by a more active process which can 

 be regarded as a modified invagination ; and they therefore consider the sub- 

 germinal cavity, into which the endoderm is pushed, as a true blastocoel. 



