62 PRINCIPLES OF EMBRYOLOGY 



there is only a minor difference between the animal and. vegetative cells, 

 to those in w^hich cleavage is confmed to a relatively tiny area at the 

 animal pole. The frog or newt are good examples of slightly unequal 

 cleavages, some of the cartilaginous fishes are the best intermediate types, 

 while the reptiles, teleosts and birds are the classical 'superficial' types. 



In the more primitive, nearly equal, members of tliis series (such as 

 Amphibia) the pattern seems to be one of bilateral symmetry, the first 

 two cleavages being vertical, and forming four cells of which one pair is 

 slightly larger than the other. The third cleavage is horizontal, but lies 

 above the equator, so that the less yolky animal cells are smaller than the 

 vegetative ones. This difference is greatly exaggerated in the more yolky 

 eggs, such as those of sturgeons. But in the most yolky eggs there is a 

 sharply different type of cleavage. The small animal cells do not gradually 

 shade off into larger and larger blastomeres ; instead, the cleavage occurs 

 only in the animal cytoplasmic region, in which all the cleavage cells are 

 of similar size, while the main mass of yolk remains completely undivided. 

 There is thus a rather sharp boundary between the cellular and non-cellular 

 parts of the egg. (There may be a few nuclei scattered in the yolk just 

 near the border of the cellular region, these being derived from super- 

 numerary sperm which enter the egg outside the sphere of influence of 

 the primary fertilising sperm.) This is the condition in reptiles and birds. 



In a very rough way, the series from non-yolky, totally cleaving eggs 

 up to very yolky, superficially cleaving types parallels the evolutionary 

 series of the vertebrates. But the parallel breaks down entirely when we 

 come to the mammals. Since it is nurtured in the uterus, the mammal egg 

 has no need of large stores of yolk, and in fact is not provided with them. 

 And in the absence of yolk, the cleavage is total and more or less equal, 

 though with no very well-defined symmetry pattern. 



2. The pattern of cleavage and the pattern of the embryo 



Since the cleavages frequently follow a defmite and orderly pattern, 

 it is perhaps natural to expect that this will be directly related to the 

 pattern of the embryo which, eventually develops. Many of the earliest 

 studies on the physiology of development aimed at discovering whether 

 this is so or not. It turns out that there is no simple, single answer which 

 apphes to all animals ; in fact this is a question to which we shall have to 

 return several times in the later discussions of the development of different 

 groups. Even the very earHest experiments, at the end of the last century, 

 showed that the subject was complicated. Driesch, in 1891, separated the 

 first two blastomeres of an echinoderm egg, and found that each gave rise 

 to a complete embryo, not to only half an embryo as would be expected 



