285 



described being due to their lobulation or even fragmentation in cer- 

 tain cases. On the other hand there is good evidence that the clea- 

 vage centrosomes are not derived exclusively either from a sperm cen- 

 trosome or from an egg centrosome but that one of these comes from 

 the egg sphere, the other from the sperm sphere (Dia, F). 



C,^>et«vNvxtW-S^ jffi E53 Nucleus I EggCkro^vo.o«« 5X^ PoUr Bod-ies 



G H 



Diagram G. Cleavage centrosomes and half spindles. Fused egg and sperm 

 spheres around nuclei (dark area). 



Diagram H. First cleavage, metaphase. 



III. Cleavage. 



I have followed the lineage and history of all the centrosomes 

 and spheres throughout the cleavage up to the formation of 48 cells. 

 In the main the history of the centrosomes and spheres during cleavage 

 is similar to that during maturation, though showing certain inter- 

 esting modifications. 



Centrosomes. In the early prophase each centrosome is a 

 minute point; in the metaphase it becomes a sphere with faintly 

 staining center; in the anaphase it becomes a large hollow sphere 

 which contains a very delicate reticulum of threads and granules 

 (Dia. B, 1 — 5). During the "rest period" this reticulum grows denser 

 and more chromatic until it resembles a small nucleus (Dia, B, 6), 

 and recalls the "spongy centrosomes" observed by R. Hertwig^) in 

 Actinosphaerium. This centrosome then becomes elliptical and two 

 larger granules appear within it near its poles ; these are the centro- 



1) R., Hertwig, Kernteilung, Richtungskörperbildung und Befruch- 

 tung von Actinosphaerium. Abh. K. bayer. Akad d. Wiss., Bd. 19, 1899. 



