12 THE DEVELOPMENT OF THE CHICK 



age or segmentation of the ovum), with which development 

 begins, involves the entire ovum. This occurs where the amount 

 of the yolk is relatively small and where it is completely inter- 

 penetrated by sufficient protoplasm to carry the planes of divi- 

 sion through the inert yolk. But where the amount of yolk 

 becomes very large, or where it is not interpenetrated sufficiently 

 by the protoplasm, the division planes are confined to the proto- 

 plasmic portion of the ovum, and the yolk remains undivided. 

 Such ova are known as meroblastic. In these ova the cellular 

 part of the ovum forms a blastodisc (germinal disc) on the surface 

 of the yolk. The ova of Amphioxus, Petromyzontidse, Ganoi- 

 dea, Dipnoi, Amphibia, Marsupialia, and Placentalia are holo- 

 blastic; those of Myxinoidea, Teleostei, Selachia, Reptilia, Aves, 

 and Monotremata are meroblastic. 



It is obvious that transitional conditions between holoblastic 

 and meroblastic ova may occur; such are in fact found among 

 the ganoids. In Lepidosteus, for instance, the quantity of proto- 

 plasm in the lower hemisphere is so slight that the division planes 

 form with extreme slowness. On the other hand, it should be 

 emphasized that the distinction between holoblastic and mero- 

 blastic ova is not so much due to amount of yolk as to the defi- 

 niteness of its separation from the protoplasm. Thus the ova 

 of some teleosts, particularly of the viviparous forms described 

 by Eigenmann, are many times smaller than the ova of Necturus 

 or Cryptobranchus among amphibia. Yet the teleost ovum is 

 meroblastic, because the protoplasm does not penetrate suffi- 

 ciently into the yolk, and the amphibian ovum is holoblastic. 



Comparison of the Germ-cells. Although it is not within the 

 province of this book to enter fully into a discussion of this ques- 

 tion, yet it should be pointed out that, in spite of the extreme 

 differences in the structure of the germ-cells, they are exactly 

 equivalent in hereditary potency, as is proved by the similar 

 nature of reciprocal crosses. Their resemblances are in fact 

 fundamental and their differences must be regarded as adapta- 

 tions to secure their union. The comparative history of the 

 germ-cells, that is a comparison of ovogenesis and spermato- 

 genesis, brings out their fundamental similarity as germ-cells. In 

 both the ovogenesis and spermatogenesis three periods are clearly 

 distinguishable, viz. : a period of multiplication, a period of growth, 

 and a period of maturation. In the period of multiplication 



