THE GAMETES 33 



that the comparatively simple maturation of the sperm is profoundly 

 modified in the differentiation of eggs (Fig. 2.1). 



The essential feature of the modification is the intercalation of a long 

 stage of growth into the sequence. This occurs, in oocytes, before the 

 first maturation division is completed; usually, in fact, the first division 

 starts, and then goes as it were into a state of suspended animation while 

 the cytoplasm and even the nucleus enlarge to many times their original 

 volume. In some eggs (most invertebrates), the maturation divisions are 

 not resumed until stimulated to do so by fertilisation. In others, such as 

 most vertebrates, the egg completes its first division, but sticks again in 

 the middle of the second until fertilised; it is only in a few types (e.g. 

 echinoderms) that both the maturation divisions are completed before 

 the eggs are shed from the ovary. 



The intercalated growth stage not only interrupts the continuity of 

 the maturation divisions, but also modifies the relation between the 

 division of the nucleus and the cytoplasm. It is as if the production of one 

 full-sized egg were as much as one oocyte can manage; if, in the middle of 

 its process of growth, each oocyte were to divide into two and then again 

 into four equal parts, it would have to produce enough substance to make 

 four eggs. The necessity for such an enormous achievement is avoided by 

 making the cytoplasmic divisions extremely asymmetrical, only a tiny 

 lump of cytoplasm being cut off from the main body of the egg, which 

 remains substantially intact. The first of the lumps is formed, in marine 

 eggs, at the end of the egg which floats uppermost. This end is known as 

 the 'animal pole' (the opposite, heavier end being the vegetative pole'). 

 The small cell produced by the first maturation division is thus known as 

 the first 'polar body'. In the second maturation division it may divide 

 again, while a second similar small body is given off from the egg. Thus 

 the maturation divisions, instead of producing four ova from one oocyte, 

 finally give rise to one ovum and three polar bodies. The latter soon 

 degenerate, and, except in very pecuHar circumstances, play no part in 

 the development of the embryo. 



During the growth of the oocyte, there is considerable activity both 

 in the nucleus and in the cytoplasm. The former enlarges greatly, becom- 

 ing a so-called 'germinal vesicle', filled with a voluminous nuclear sap. It 

 is one of the great gaps in our knowledge of oogenesis that we know so 

 little about the constitution of this sap, except that it is rich in sulphydryl- 

 containing proteins (Brachet 1952^; Brown, Callan and Leaf 1950)- 

 The chromosomes, arrested at some stage in meiotic prophase, usually 

 tend to enlarge and become less densely staining. In highly yolky eggs 

 which have a long growing period, this expansion of the chromosomes 



