SMALLWOOD : MATURATION OF HAMINEA SOLITARIA. 309 



cleavage asters to be exclusively associated with either of the two pro- 

 nuclei. Moreover, Linville apparently draws his conclusions from the 

 later rather than the very earliest stages. It is probable that more ex- 

 tended observations, and especially the study of stages earlier than those 

 reproduced by him, would reveal a more definite relation between the 

 earliest traces of the cleavage asters and the pronuclei. Evidently, 

 then, the more recent investigations on Mollusca indicate that each pro- 

 nucleus is associated in origin with one and only one of the cleavage 

 asters. 



5. Summary. 



First Maturation. Ovocyte. The cytoplasm of the young ovocyte is 

 of homogeneous appearance. When deutoplasm begins to form in it, 

 vacuoles appear in the nucleolus. These increase in number and size ; 

 by their confluence the nucleolus comes to contain a single large vacuole. 

 The nucleus contains both basichromatin and oxychromatin granules. 



Centrosome. The centrosome in the earliest condition observed is 

 composed of five bodies ; these unite into one before the egg is laid. 



The cortical layer of the " sphere " enveloping the centrosome is 

 finely granular, free from yolk spheres, and destitute of the reticulate 

 structure of the cytoplasm. 



The centrosome, at first homogeneous, becomes diflFerentiated into 

 centriole and centroplasm. The centroplasm is, by definition, never 

 traversed by astral rays. At the same time the sphere-substance is 

 differentiated into a cortical and a medullary layer. During the forma- 

 tion of the centroplasm its periphery has an irregular outline, which 

 continues until the centrioles divide. 



Chromosomes. The chromosomes first appear in the form of deposits 

 on the walls of chromosomal vesicles which are derived chiefly from the 

 linin threads of the germinative vesicle. When the chromosomes have 

 attained considerable size, the walls of the chromosomal vesicles disappear. 

 The spireme of the germinative vesicle does not give rise to the chromo- 

 somes by either segmentation or splitting of its substance ; but there 

 intervenes between the two a stage in which the chromatic substance, 

 reduced to a minimum, can be detected only as a partial investment of 

 the linin network of the disintegrating germinative vesicle The indi- 

 viduality of tlie chromosomes cannot, therefore, be demonstrated in 

 Haminea. 



Second Maturation. Tlie eggs of Haminea form the second maturation 

 figure by either one or the other of two somewhat different, but appar- 



