vol.3.1 Robertson. — Embryonic Fission in Grisia. 14.'; 



is still tin perforated. Finally, although empty ovieells are 

 remarkably scarce, yet in one instance at least, a complete series 

 was obtained which possessed neither larva?, nor embryos, the 

 interior containing nothing but a fine network and some degen- 

 erated cells. The evidence seems to be conclusive, then, that the 

 whole of the primary embryo is converted into larvae. 



The number of larva' to which a colony of Grisia gives rise is 

 probably not less than is produced by other bryozoa although Grisia 

 produces few mature eggs. As far as the evidence from my 

 observations is concerned all the larva' found in the ovieell, arise 

 from one egg. Both Harmer and Calvet, however, believe 

 they have evidence that more than one ovum may develop simul- 

 taneously within a single ovieell. Harmer ('97, PI. !). Fig. 25), 

 represents two young embryos whose blastomeres are still separ- 

 ated, which he considers are the result of the development of 

 two eggs. While this may he true, there is a possibility that 

 the conditions presented may have resulted from the blastomeres 

 of the two-cell stage of a single ovum having become so 

 widely separated that each has gone on to develop into a separate 

 embryo. The numerous recent experimental demonstrations of 

 the power of independent development possessed by the blasto- 

 meres, and this too, in ova whose blastomeres normally retain 

 their connection with one another, renders this hypothesis 

 the more probable. Calvet figures a similar condition (PI. 10, 

 Fig. 15) which he considers affords undoubted evidence 

 of the preseuce of two ova and of their simultaneous develop- 

 ment within a single ovieell. Here again the facts may be 

 differently interpreted. The two embryos may represent the 

 individual development of two blastomeres which had become 

 separated in the two-cell stage and had not reunited, or it may 

 be an instance of a condition similar to what occurs in Grisia 

 occidentalis. The two large masses, the two so-called primary 

 embryos, may be two secondary embryos, and the smaller 

 masses arising from these, may be tertiary embryos. The 

 production of tertiary embryos is reported for Lichenpora and 

 Tubulipora, but has not been previously found in Grisia. In the 

 species in which it undoubtedly occurs, Grisia occidentalis, there 

 is more or less variation, and it will not be surprising to find it 

 in all species of the genus. 



