hargitt: pennaria tiarella and tubularia crocea. 205 



ruptured. A large centrosphere with astral radiations is present, 

 but no central body could be found. Cytoplasmic division is con- 

 siderably delayed, the second nuclear division being finished before 

 the first cleavage furrow has cut half through the egg. 



B. Tubularia crocea. 



1. Oocytes. — The primordial germ cells divide mitotically to 

 form the oogonia, and the latter by mitosis give rise to the oocytes. 

 The daughter chromosomes resulting from the last oogonial division 

 lose their individuality, and at this time occurs a differentiation into 

 food cells and egg cells. In the former the chromatin of the nucleus 

 becomes scattered in large granules along a delicate linin reticulum, 

 which is limited to the outer half of the nucleus, but is connected by 

 linin fibres with the central nucleolus. These cells have not, perhaps, 

 lost their power of becoming egg cells, but most of them serve as food 

 for other oocytes. In the oocytes which form egg cells at once, the 

 chromatin forms a definite spireme, which gives rise to more or less 

 distinct loops. The loops assume a definite polar arrangement, their 

 open ends being attached to the nuclear membrane. This apparently 

 represents the synapsis stage, and reduction of chromosomes seems to 

 occur at this time, though how the reduction is actually accomplished 

 could not be determined. 



a, Germinative vesicle. — The polar arrangement of the chromatin 

 is soon lost, and the oocyte begins to grow rapidly. The loops become 

 granular and more delicate and may undergo a longitudinal splitting, 

 but the evidence on this point is too scanty to allow any conclusions 

 to be drawn. As growth continues the germinative vesicle shows no 

 further sign of chromatin loops, and toward the end of growth exliibits 

 only a mass of fine granules which select plasma stains. 



b. Nucleolus. — The nucleolus of the oocyte is plasmatic at all 

 times. It increases in size for a short time in the young oocyte (per- 

 haps by the absorption of nuclear sap), but as soon as the oocyte begins 

 to grow the nucleolus may begin to decrease in size, or this decrease 

 may not begin till a much later period. This decrease is accomplished 

 in two ways: (1) Liquid substances pass out of the nucleolus and along 

 the linin reticulum to become incorporated in the reticulum, presum- 

 ably with the chromatin; (2) actual fragmentation may occur in the 

 early stages, and in the later growth it always occurs. The fragments 

 either dissolve in the nuclear sap or become arranged along the nuclear 

 reticulu " and are eventually transformed into, or absorbed by, the 



