414 CASWELL GRAVE 



During the summers of 1900, 1901, 1902, and 1904, while 

 at the Fislieries Laboratoiy at Beaufort, North Carohna, I 

 was able to follow the development of living eggs and larvae 

 of this species and to preserve an abundance of material from 

 which it is now possible to complete the account of its develop- 

 ment and to confirm or correct conclusions which, as first pub- 

 lished, rested upon insufficient observation. 



I am now able also to show that the organization of the egg 

 of Ophiura, although seemingly different from that of other 

 echinoderms, is in reality fundamentally the same. Since the 

 apparent peculiarities in the organization and development of 

 the egg of Ophiura are possibly to be correlated with the un- 

 usually large amount of yolk with which it is filled, a fairly com- 

 plete account of the distribution, arrangement and form of 

 the yolk within both the unsegmented egg and the cells of the 

 early larva, and of the effect upon development and develop- 

 mental processes traceable directly or indirectly to the yolk, 

 is included. Certain observations on the breeding activities 

 of the species which supplement those of the former paper are 

 appended. 



THE EGG 



The mature egg of Ophiura brevispina has a diameter of 

 approximately 0.3 mm. and is therefore about seventy-eight 

 times the volimae of the egg of Ophiocoma echinata, the latter 

 being not far from the minimum size of eggs produced by ophiu- 

 rid species in general (text figs. 3, D and C) The great differ- 

 ence in the size of the eggs of these two species seems to be chiefly, 

 if not exclusively, due to a difference in yolk content and not 

 to a difference in volume of ground substance. 



The yolk material of the egg of Ophiura, like the yolk material 

 of all echinoderm eggs, is distributed uniformly throughout the 

 cytoplasm in the form of minute spherules and does not be- 

 come stratified or localized within the egg either before or after 

 maturation. The yolk spherules are practically uniform in 

 size and, in the same brood of eggs, in color also, and they give 

 to the eggs their characteristic tint. There is a striking varia- 



