REJUVENESCENCE IN EMBRYO AND LARVA 419 



increase there is no increase, but rather a decrease, in total cyto- 

 plasmic volume, for the nuclear substance is formed at the expense 

 of the cytoplasm or of substances contained in it; consequently 

 the relative increase in nuclear substance is somewhat greater than 

 the absolute. According to Erdmaim ('08), the nucleoplasmic 

 relation, that is, the volume of the nucleus in relation to the volume 

 of the cytoplasm, undergoes very great increase from the four-cell 

 stage to the gastrula in the sea-urchin, and the volume of the 

 chromosomes, in relation both to cell volume and to nuclear volume, 

 also increases during this period. Conklin ('12), in a study of the 

 moUusk Crepidula, also finds an increase in total nuclear volume 

 during cleavage, though by no means so great as that found in the 

 sea-urchin. 



The change in the nucleoplasmic relation during this period is 

 evidently in the reverse direction from that which it underwent 

 during the growth period of the gametes. Undoubtedly the 

 increase in relative nuclear volume during early development is, as 

 Conklin points out, an important factor in the acceleration of meta- 

 bolic activity, but it is not the only nor even the primary factor, 

 for the acceleration may begin before the nuclear increase, and 

 under other conditions acceleration of metabolism may occur with- 

 out such increase. The increase in nuclear volume is an indication 

 rather than a cause of the metabolic changes which the embryo is 

 undergoing during this period. Moreover, as regards the sperma- 

 tozoon, entrance into the egg constitutes a sudden and enormous 

 increase in cytoplasmic volume, yet the spermatozoon undergoes 

 regressive changes as well as the egg. The general significance of 

 the nucleoplasmic relation for the problem of age is considered in 

 chap, xvi (see also pp. 284-86). 



In most animal eggs the cytoplasm contains more or less fatty 

 substance — the yolk^ — in the form of granules, droplets, or large 

 masses, and in such eggs the most conspicuous cytoplasmic change 

 during the early stages of development is the gradual disappearance 

 of this yolk. But even in eggs which contain no visible yolk the 

 cytoplasm becomes more homogeneous in appearance, and cyto- 

 plasmic strands, granules of various sorts, and other structural 

 features of the egg disappear wholly or in part. At the same time 



