434 CASWELL GRAVE 



for the phylum, is not to be regarded as a gastrula, but that it 

 is the modified blastula of species producing eggs heavily charged 

 with yolk material. Primitively the blastula of coelenterates 

 may have developed into the gastrula stage by a typical process 

 of invagination, but as the yolk content of the egg increased 

 gastrulation became more and more hindered until, finally, 

 it could only take place following a segregation of a portion of 

 the yolk of the entoderm cells into the segmentation cavity 

 (Urticina). When the yolk content of the egg still further 

 increased, the entire space of the segmentation cavity was re- 

 quired to contain it, and, in response to such a condition, which 

 is the common one among coelenterates, the planula larva was 

 developed in which the entoderm cells take their definitive 

 position not by invagination but by delamination or migration. 



CORRELATION BETWEEN YOLK CONTENT OF EGG AND 

 DURATION OF DEVELOPMENT 



Various echinoderms differ greatly in the size of their eggs 

 and also in the duration of their larval periods of development, 

 and a considerable mass of embryological data seems to indi- 

 cate that, in the absence of brooding of the offspring by the par- 

 ent, a fairly definite correlation may exist between these two 

 factors such that any increase or decrease in the first is accom- 

 panied by a corresponding inverse change in the second. 



The existence of some sort of correlation between yolk con- 

 tent of egg and character of development throughout the animal 

 kingdom is a matter of general observation and many references 

 to it are to be found in papers on embryology. Students of 

 embryology expect to find a modified development in all cases 

 of yolky eggs and a developmental period of shortened duration 

 in all cases in which such eggs are not in some way protected 

 or cared for during development. 



As stated above, the eggs of Ophiura brevispina (0.3 mm. in 

 diameter) and Ophiocoma echinata (0.07 mm. in diameter) 

 probably represent the extremes in a series of eggs of ophiurids 

 (fig. 3 C and D), arranged as to size but, when the eggs of 



