XX NOTES ET REVUE 



maturation spirKile. In the laller, (lie U-shaped chromosomes lie 

 Iransrerseli/ lo the lengtli of tlie spindle, willi llio curve of the U 

 touchiuf;- its eqiialor and willi the points directed oiilward (Fig. 2). 

 The ciu'oinosouies then beconie divided, each at th(! curve of the U, 

 where the spindle libres are attached. This is the conipletion of the 

 process of longitudinal splitting, and is therefore an équation 

 division. Thus, in Phascolosoma, the usual order of maturation divi- 

 sions is reversed. It is an example of what Kohschelt and Heider * 

 hâve called a " Prsereductionsdivision. » 



The sperm nucleuswith its large astrosphere awaits in the middle 

 of the egg the female pronucleus. The aster of the latter, which 

 has been derived from the second maturation spindle, be- 

 comes smaller, but does not completely disappear. In the middle 

 of the enlarged astrosphere of the sperm nucleus a minute astro- 

 sphere appears, which contains the centrosome and is immediately 

 surrounded by prominent curved astral fibres. When the two pro- 

 nuclei meet between the animal pôle and the centre of the egg, 

 each is accompanied by its own aster. The position of the respec- 

 tive astrospheres and the différence in size between the asters 

 which surround them make their identification easy, until equa- 

 lity in size is established between the two. When this occurs, they 

 hâve already moved to the opposite pôles of the plane of contact 

 between the two pronuclei. 



Segmentation of the Egg. The most striking features of the 

 cleavage are : 



I. The large size of the first quartette of " micromeres », which 

 in the three quadrants A, B and C (ventral in the embryo) some- 

 what exceed in size the « inacromeres » or blastomeres at the 

 vegitative pôle. 



II. The alternating directions of the spindles of segmentation in 

 successive stages up to 48 cells, and in certain régions of the egg 

 still further, which accord completely with the usual type of spiral 

 cleavage. 



III. The présence at tlie active pôle in the 48-celled stage of the 

 rosette, cross and intermediate cells, which are characteristic of 

 Annelids and represented also in Molluscs and other groups 

 (Fig. 3). The rosette and intermediate cells, which are ail of nearly 



' Lehrbuch der vergleichenden Entwicklungsgeschichte, Allgemelner Theil, Zweite 

 Lieferung (lena, 1903). 



