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 blastodise (germinal disc) on the surface 
of the yolk. The ova of Amphioxus, Petromyzontidxe, 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 sight 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, vet 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 
