436 CASWELL GRAVE 



an abrupt metanioipliosis, are characteristic of those species 

 which produce large yolky eggs. 



Eggs of the yolky type contain an amount of reserve material 

 and energy sufficient for all of the processes of development, 

 hence larvae which come from such eggs develop rapidly into 

 the adult condition, but, in the so-called alecithal type, the 

 supply of available nutiitive material in the egg becomes exhausted 

 before development has proceeded far and larvae developing 

 from such eggs are therefore under the necessity of spending 

 a very considerable portion of their inherited energy in the 

 acquisition of an additional sujiply of material with which to 

 complete their development and for perfecting the larval me- 

 chanisms essential to their struggle for existence as independent 

 organisms. The larval forms live in an environment totally 

 different from that of the adult, hence structures and functions 

 different from the corresponding ones of the adult are required 

 by the larvae for capturing, digesting and assimilating food, 

 for locouTotion, for escaping enemies, etc. 



It may be assumed that the time required for developing, per- 

 fecting and maintaining these extra-larval structures and func- 

 tions is, in some way, added to the period required for the purely 

 developmental differentiations and that the periods of larval 

 development are thereby lengthened to degrees proportional 

 to the complexity of the extra-larval structures and activities 

 for which material and energy must be provided. 



It is perhaps possible therefore roughly to indicate the cor- 

 relation which exists between the yolk content of eggs and the 

 periods of time required for their development, by a curve, 

 such as that reproduced in text figure 4, in which the spaces 

 along the axis of ordinates represents units of egg-diameter and 

 those along the axis of abscissae represent unit periods of time. 



This four-point curve, constructed on the basis of measure- 

 ments and observations of the eggs and developmental histories 

 of Ophiura (Grave), Ophiothrix (McBride), Mellita (Grave) 

 and Toxopneustes (Tennent)- cannot be considered an accurate 



^ Unpublished data kindly su|)])licd hy Dr. D. H. Toniiont. 



