310 bulletin: museum of comparative zoology. 



ently normal, processes. Only one of the processes occurs in eggs laid 

 by a given individual. Of these two processes, the one which I have 

 called the shorter or direct is the simpler and more like that known to 

 exist in most other animals. In it the chromosomes neither lose their 

 individuality nor pass through the network stage characteristic of tlie 

 so-called quiescent nucleus. They are never found far removed from the 

 animal pole of the egg. In the longer or indirect process the young 

 achromatic figure is formed at a distance from the animal pole, and with 

 the immediately surrounding cytoplasm moves peripherally. The chro- 

 mosomes of the inner half of the first maturation figure in this process 

 pass into the state known as the quiescent nucleus, characterized by the 

 presence of a reticulum. While in this stage they divide in preparation 

 for the formation of the second polar cell. 



Some of the chromosomes become vesicular (Plate 6, Fig. 32 ; Plate 7, 

 Figs. 40, 43) in both processes, but this condition is not assumed at any 

 definite time. 



The central spindle arises at the time of the formation and moving 

 apart of the two new centrioles ; later it is dissolved. The centrosomes 

 move far apart, and a new spindle is formed between them. The cen- 

 tral spindle is an outgrowth from the chromosomes ; but the mantle fibres 

 are formed from the cytoplasm. During the separation of the centro- 

 somes there is a marked change in the distribution of the cytoplasm in 

 the egg ; this indicates that the total contents of the ovum are affected 

 by the movement of the centrosomes, or that both owe their activity to 

 a common cause. The two processes, the direct and the indirect, are 

 practically identical from the time of the metaphase onward. 



During the second maturation phenomena the centrosomes pass through 

 a metamorphosis similar to that which occurs during the first maturation 

 process ; but in the indirect process there is a noticeable lack of synchro- 

 nism in the development of the two centrosomes in relation to each 

 other and to the metamorphosis of the chromosomes. 



The centrosome at the deep end of the second maturation spindle 

 disappears at the close of maturation. The corresponding chromosomes 

 pass into a vesicular state and finally fuse to form the female pronucleus. 

 The sphere-substance around the female pronucleus also becomes un- 

 recognizable before cleavage begins. 



In the first maturation the chromosomes divide transversely, but in 

 the second maturation it is difficult to ascertain whether the division is 

 transverse or longitudinal. A quantitative, but not a numerical, reduc- 



