FERTILIZATION AFTER INITIATION OF DEVELOPMENT. 273 



abnormal and much smaller swimming larva than the normal. 

 On the other hand, the writer has watched for long periods of 

 time the half- or quarter-eggs found in the dish at the time of 

 insemination but has not found for certain a single one of these 

 that produced a membrane or has segmented as a result of 

 fertilization. But if one should only casually observe the eggs 

 following such a treatment there is no way to distinguish between 

 the separated blastomeres of an egg that has been fertilized and 

 has segmented into two blastomeres that have become separate, 

 and one of the half- or quarter-eggs that was present at the time 

 of insemination. Of importance also is the fact that the char- 

 acter of a swimming larva of reduced size found in these inseminated 

 cultures is no criterion that would indicate from u'kat kind of an 

 egg it ivas derived. As before stated, swimming larvae of all 

 sizes and of most any external shape assumed by protoplasm 

 can be found in dishes, the eggs of which have been exposed to 

 the hypertonic treatment alone. How then may we conclude 

 that a half- or quarter-sized larva obtained in such a mixed 

 culture after insemination, was derived from a half or quarter- 

 sized egg by fertilization? The only possible way in which the 

 writer could know the exact conditions of the egg from which 

 the swimming larvse came was by the very laborious method of 

 isolation of individual eggs, and this method was adopted as a 

 last resort. 



Four or five hours after an exposure to the weaker concentra- 

 tion of hypertonic sea-water one often finds that 2 per cent, or 

 3 per cent, of the eggs have segmented once, producing two 

 equal blastomeres that are separated from each other but remain 

 enclosed by the thin transparent jelly layer of the egg. By 

 means of a very fine capillary pipette connected with a flexible 

 rubber tube held in the mouth, these two separate cells, or half- 

 eggs, were isolated from the cultures while in focus under a 

 binocular microscope and collected into a small dish. Thus it 

 was definitely known that only blastomeres of one kind were 

 present. These isolated half-eggs could then be observed and 

 be inseminated in a pure culture, and hundreds of them were 

 isolated and studied in this manner, enabling the writer to deter- 

 mine the following points: 



