262 BULLETIN: MUSEUM OF COMPAEATIVE ZOOLOGY. 



My first plan was to make a detailed study of cell lineage ; but Ha- 

 minea showed so little variation from Crepidula, Umbrella, Planorbis, 

 and other Mollusca that it did not seem desirable to give to this problem 

 the time necessary for its completion. The present paper is confined to 

 the cytological changes that take place during maturation, fertilization, 

 and the early cleavages. 



Kotwithstauding the great volume of cytological literature, there arc 

 still a number of unsettled questions relating to the early stages of 

 development. Experimental study has thrown doubt on some accepted 

 conclusions and suggested new questions. Before we can adopt the new 

 conclusions, we should find some confirmation of them from a study of 

 development under normal conditions. The question of the individuality 

 of the chromosomes, the changes and the permanency of the centrosome, 

 the precise nature of fertilization, — these and other problems are still 

 unsettled. 



II. Collection of Material and Technique. 



The opisthobranch Haminea solitaria is common along the Atlantic 

 coast from Cape Cod to Florida. The species that occurs at Wood's Hole 

 is undoubtedly identical with those that have been described from various 

 other places along the coast, the differences in description being due 

 chiefly to the age of the animals examined. Haminea solitaria is not 

 commonly seen except in the breeding season, which at Wood's Hole ex- 

 tends from June to September. During this time the animals are found 

 in favorable places in large numbers. The rest of the year they live in 

 deep water. 



Sometime, usually between the middle of June and the middle of July, 

 Haminea migrates from the deep water into shallow ponds and lagoons, 

 where the eggs are deposited on the mud, eel-grass, stones, sticks, etc. 

 The development takes place so rapidly that to secure eggs in early 

 stages of development, it was found necessary to collect the animals be- 

 fore the eggs had been laid. During the breeding season Haminea is 

 not very sensitive to artificial conditions, so that it was possible to keep 

 the animals under direct observation, although they died soon after egg- 

 laying. During tlie summer of 1900, I was fortunate in collecting over 

 forty adult individuals before copulation took place. Thus having the 

 animals under direct observation and separated from all other forms, 

 and knowing when copulation took place, I was able to secure eggs about 

 the identity of which there could be no question. By acquiring a large 



