304 McMURRICH. [Vol. IV. 



study of more abundant material. Those ova which were fertil- 

 ized I did not succeed in rearing up to the time of formation of 

 the mesenteries, owing probably to ignorance of the proper food 

 which they require at this period. I had hoped to be more suc- 

 cessful during the summer just past, but my endeavors to obtain 

 ova were entirely fruitless, though spermatozoa were plentiful. 



I. The Segmentation and Formation of the Germ-layers 



IN Metridium. 



In his classic monograph on the development of the Actinians 

 Lacaze-Duthiers ('72) advances the suggestion that these forms 

 are hermaphrodite. Later observers have not succeeded in 

 confirming this opinion in so far as the Hexactinians are con- 

 cerned, though the coexistence of ova and spermatozoa has 

 been found in certain Zoantheae and Ceriantheae. I have never 

 found in Metridium, or in any of the numerous other Hexac- 

 tiniae I have examined, any trace of hermaphroditism, and 

 believe that it may be accepted as a rule that the Hexactinians 

 are bi-sexual. Dichogamy may possibly occur, but the evidence 

 seems strongly against even this. 



The immature ova of Metridiwn lie closely packed together 

 in the mesogloea of the gonophoric mesenteries, mutual pres- 

 sure compelling them to assume irregular shapes. When set 

 free by teasing, the younger ova are almost spherical, while the 

 older ones (PL XIII, Fig. i) are somewhat irregular, the majority 

 possessing a more or less elongated process extending out from 

 the circumference at one point, sometimes in the neighborhood 

 of the nucleus, sometimes distant from it. The nucleus is large, 

 with a single large nucleolus, and is always situated eccentri- 

 cally, lying as a rule nearest that pole of the ovum which is 

 adjacent to the free surface of the mesentery. The peculiar 

 filamental apparatus which the Hertwigs have described in con- 

 nection with the immature ova of Adamsia parasitica (yg) and 

 other forms ('82), I have never observed in Metridium. 



The mature eggs are perfectly spherical, pinkish in color, 

 and opaque with small granules of food-yolk. They measure 

 o. 1 24 mm. to o. 1 59 mm. in diameter, and each ovum is surrounded 

 by a delicate membrane in which no perforations are visible. 

 No nucleus can be detected in the fresh ova after their extru- 



