2IO BIOLOGICAL LECTURES. 



explicit, if the division of the mature egg-cell is inhibited 

 because its centrosome degenerates, and is subsequently re- 

 sumed because a new centrosome is introduced, it is fair to 

 infer that the division of a blastomere, a tissue-cell, or an 

 oocyte is also conditioned upon the presence of the centro- 

 some. For example, the prototroch-cells in the annelid larva, 

 or the neuroblasts in the vertebrate embryo, cease' to divide 

 because the "centrosomes by which division is initiated" have 

 degenerated. Again, the cells by whose rapid division lost 

 tissues are regenerated resume their mitotic activity upon the 

 acquisition of a centrosome. 



With this explanation in mind, let us return to a more 

 critical examination of the phenomena of mitosis in the germ- 

 cells at the close of the life-cycle; viz., to the division of the 

 primary and the secondary oocytes and the mature ^g^. 



It is noticeable, in the first place, that among different species 

 of animals the mitosis is not always suspended in the same gen- 

 eration of cells. Accordingly, the eggs of the various species 

 may be arranged in five classes : first, those in which the pri- 

 mary oocyte remains with germinal vesicle intact until the 

 sperm enters (for example, Thalassema, Nereis, Fig. 4, A)\ 

 second, those which, if deposited in sea-water, remain with the 

 first maturation amphiaster in the metaphase (for example, the 

 annelid Chaetopterus, Fig. 4, B) ; third, those in which the sec- 

 ondary oocyte awaits the spermatozoon (for example, the frog, 

 Axolotl, lamprey. Fig. 4, C); fourth, those in which the matu- 

 rated Qg^, after the formation of both polar globules, awaits 

 the sperm (for example, the sea-urchin. Fig. 4, D) ; and fifth, 

 those (parthenogenetic eggs) which begin the subsequent cycle 

 of divisions without being fertilized. 



Let us grant for the moment that the parthenogenetic egg 

 continues to divide because its centrosomes do not degenerate, 

 and that the fertilized Q.gg of the sea-urchin divides because 

 the entering sperm brings in the centrosomes which organize 

 the machinery of mitotic division. It is difficult to apply the 

 same principle to the eggs of the first, second, and third classes, 

 i.e., to those which do not complete the maturation divisions 

 until the spermatozoon enters, for the amphiasters of the matu- 



