DEVELOPMENT OF EGG FRAGMENTS IN CEREBRATULUS. 133 



much smaller. It is remarkable that the development of the iso- 

 lated blastomeres differs very widely from that of fragments of the 

 period preceding. From a comparison with Series D it seems 

 likely that the result may be different according to the period of 

 cutting /. e., before or after the disappearance of the cyto- 

 plasmic bridge (cf. Series D, eggs B and C) and the pressing to- 

 gether of the blastomeres. In any event, further careful exami- 

 nation of the egg of this stage with reference to the question of 

 egg specification is very desirable. 



CONCLUSION. 



From the above series of experiments we have seen that, if the 

 cutting-off of a portion of cytoplasm is done before maturation, it 

 only slightly affects the normal development ; but if the opera- 

 tion is done at the first maturation stage the formation of the 

 organs of the pilidia is considerably affected, and still more if it 

 is done at the time of fusion of the germ-nuclei. We cannot, 

 therefore, escape the conclusion that there must take place some 

 progressive changes in the general make-up of the egg during 

 the period extending from the time of dissolution of the germinal 

 vesicle to the fusion of the germ nuclei. This period falls into 

 two subdivisions, the first extending from the fading of the 

 germinal vesicle to the metaphase of the first polar mitosis, and 

 the second from the growing period of the sperm-nucleus to its 

 conjugation with the egg-nucleus. The importance of these two 

 periods has been correctly perceived by Delage (1901), Fischel 

 (1903) and recently by Professor Wilson (1903). It is hardly 

 necessary to state here that, although these two periods can 

 be artificially separated by delaying fertilization, yet, under natural 

 conditions, there is no pause between them, as the spermatozoon 

 enters the egg before its maturation. 



Let us next see what visible changes take place in the egg 

 cytoplasm as the result of the above two acts, i. e., the dis- 

 appearance of the germinal vesicle and the entrance of the 

 spermatozoon in the matured cytoplasm. As soon as the ger- 

 minal vesicle breaks up, the nuclear fluids flow out and may 

 diffuse through the egg. At this time currents may be formed 

 carrying the egg cytoplasm from one spot to another. Although 



