326 H. H. NEWMAN 



forming any fertilization membrane, begin cleavage. In nor- 

 mally fertilized eggs cleavage begins within about two hours 

 after maturation, if insemination occurs at the time when the 

 majority of the eggs have undergone maturation. The partheno- 

 genetic eggs are therefore about three hours slower to begin 

 cleavage than are the normally fertilized eggs. This very pro- 

 nounced retardation involves a period at the very beginning of 

 ontogeny, when any serious retardation must of necessity exert 

 a telling influence upon subsequent development. The effects 

 of this influence vary in severity in different eggs, presumably 

 in accord with variations in their physiological condition. In 

 the following paragraphs we shall deal with a considerable list of 

 observed conditions, which are arranged in the order of success 

 in development, beginning with cases in which the effects of 

 early retardation have been most pronounced and ending with 

 those cases in which the larvae show the most complete recovery 

 from an early inhibition. 



a. Some eggs begin, but are unable to complete, the first cleavage. 

 It is quite common to find eggs with two nuclei and a cleavage 

 furrow about half-way across the egg. Such eggs when kept 

 under observation for several hours show no change and undergo 

 cytolysis within the next twelve hours. In this case the eggs 

 evidently have been so profoundly inhibited that only a spark 

 of developmental energy remains — a spark which flares up mo- 

 mentarily, but dies out before the first cleavage is complete. 



h. Other cases were observed in which, after the successful 

 completion of the first cleavage, one blastomere only was able 

 to continue cleavage. The other blastomere remains for many 

 hours uncleaved and is sometimes partially surrounded by the 

 cells resulting from the cleavage of the other cell. This looks 

 somewhat like a case of epibolic gastrulation. Occasionally the 

 cells of the cleaving blastomere round up into a sort of soHd dwarf 

 blastula and become released from the vitelline membrane as a 

 swimming larva. This has been observed several times, and was 

 especially interesting in those cases where such an active larva 

 has had to tow about through the water the inert uncleaved 

 blastomere, which has been invaded by parasitic protozoa. 



