388 TRANSACTIONS OF THE [dEC. 13, 



The eggs were taken from the body-cavity and were fertilized 

 almost immediately on being brought into sea-water. At inter- 

 vals of five minutes stages were- preserved in both Boveri's 

 picro-acetic acid and in acetic sublimate. Only the former 

 series proved of much value. The following is concerned chiefly 

 with the cytoplasmic elements. 



During the earlier stages the eggs usually present a peculiar 

 elongated or flattened form (Fig. 1).* Whether this indicates 

 active changes of shape, such as are known to occur in some 

 eggs, it is impossible to say, as living eggs have not been 

 studied. 



The just fertilized egg contains a large nucleus with a promi- 

 nent nucleolus, and the cytoplasm is uniform in structure, con- 

 taining great numbers of small yolk-granules. There is no evi- 

 dence of a centrosome or of any radiating structure of the 

 cytoplasm. 



The first indication in the nucleus of approaching karyokinesis 

 consists in an apparent increase in the amount of tingible chro- 

 matin and the disappearance of the nucleolus. The nuclear 

 membrane begins to disappear, resolving itself into the plas- 

 matic network, and the chromatic substance is left free in the 

 cytoplasm, usually at the side of a small cavit}^ which contains 

 the nuclear liquid. As this is soon absorbed, the cavity disap- 

 pears. 



Simultaneously with the disappearance of the nuclear mem- 

 brane, about fifteen minutes after fertilization, the first traces of 

 centrosome and asters which I have been able to find make their 

 appearance. Two very small but distinct centrosomes are seen 

 (Fig. 1), each surrounded by an area of deeply staining cyto- 

 plasm, and around this appears an extremely delicate and faint 

 cytoplasmic radiation extending only a short distance. There 

 is no ti-ace of a centi*al spindle, nor has evidence been found that 

 the two centrosomes arise by the division of one. They are 

 often much farther apart than in the egg figured. 



The centrosomes increase in size and the astral rays become 

 longer and much thicker. Their ends can be seen to branch 

 and terminate in the cytoplasmic network. The region between 

 the two centrosomes is occupied by a spindle, formed by the 

 ra^^s which extend from each center toward the other. These 

 are often sinuous, and in some cases can be seen to be connected 

 by cross-branches. A section of the egg at this stage is shown in 

 Fig. 2.f Besides the spindle lies the chromatin, and the nuclear 



*The small yoke granules which abound except in the region of the asters and 

 spindle are not shown in the figures. All figures were drawn with the aid of the Abb<5 

 camera. 



t In this figure the thick egg-membrane is shown. 



