FERTILIZATION AFTER INITIATION OF DEVELOPMENT. 275 



whole egg and possibly also from an egg, one half of which 

 may have become separated after the first cleavage. 



The fact that among these 24-hour cultures of eggs, insemina- 

 tion is not usually followed by a perceptible membrane, may 

 lead one to consider the second division of an egg as a division 

 of two half-eggs that have segmented since insemination but 

 not as a result of it. If these first two blastomeres from the 

 cleavage of an egg are separated and each divides again, how is 

 one to judge whether this is the real condition or that perhaps 

 they may represent the fertilization of a half-egg? Decidedly 

 not by periodical examination, but only by continual observa- 

 tion, if in a mixed culture, or by isolation; and as has just been 

 pointed out, hundreds of isolated blastomeres have been in- 

 seminated in pure culture but not a single swimming form has 

 been obtained from these, that have remained at room tempera- 

 ture seven hours between the time of hypertonic treatment and 

 insemination. 1 Whether this explanation will serve to har- 

 monize the differences of opinion or whether there is an individual 

 variation between the fundamental processes of development in 

 the two different species of sea.-urchin eggs, the writer is unable 

 to say. 



But to obtain a further insight to the observable effects of the 

 addition of sperm to eggs that have been caused to start their 

 developmental processes, eggs from a large number of experi- 

 ments have been preserved and sectioned and the cytological 

 results are presented briefly in a following section. 



2. Fertilizin and Fertilization. 



In a former paper the writer considered in some detail the 

 relation of the presence of the sperm agglutinating substance, 

 fertilizin, liberated from the normal eggs of arbacia into sea- 

 water in which they have been standing, and the capacity of 

 eggs for fertilization, both normal eggs and eggs that had been 

 treated with various reagents. It is not the purpose of the 



1 Some time must be allowed before insemination because of the fact that some 

 of these blastomeres continue to divide before insemination; even seven hours does 

 not entirely eliminate this source of confusion, for a blastomere may divide after 

 that length of time. Developmental changes are still going on, but at a very 

 slow rate. 



