906 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



A 



FIG. 312. Stages in the segmentation of the egg (after Wilson). 

 The drawings were made from sections of eggs of the sea-urchin, 

 Tox&pneustes variegatus, Ag. 



A. Beginning of the formation of the amphiaster. The nuclear 

 membrane has disappeared at the two poles of the spindle-shaped 

 nucleus. Within the nucleus a distinction between the chromatic 

 and the achromatic substance is being made, the former existing as 

 irregular rod-shaped bodies lying at the centre, the latter as delicate 

 filaments extending irregularly from pole to pole. The asters are well 

 marked. 



B. The nuclear membrane has wholly disappeared. The chro- 

 mosomes are clearly defined and aggregated in the centre of the 

 spindle to form the equatorial plate. The achromatic filaments of 

 the spindle are well marked. The connection of the astral rays with 

 the cytoplasmic reticulum of the egg is shown. 



c. Each chromosome has become split into two, and the latter, 

 cA, are being pulled toward the poles. 



1 Th. Boveri : loc. cit. 2 E. B. Wilson and A. P. Mathews 



body, the centrosome, and 

 around the latter the fila- 

 ments of the cytoplasm of 

 the egg arrange themselves 

 in the form of a star, the 

 whole body being known 

 as the sperm-aster (Fig. 310,. 

 B). We have previously 

 recognized such a structure 

 in the ovum at the time of 

 maturation, and have found 

 it functional in the forma- 

 tion of the polar bodies; 

 after maturation it disap- 

 pears. The sperm-aster 

 accompanies the sperm-nu- 

 cleus, becomes gradually 

 enlarged, and finally comes 

 to lie, a large and promi- 

 nent body, beside the seg- 

 mentation nucleus. Its 

 origin, or, more exactly, 

 the origin of its centre- 

 some, has been greatly 

 disputed, and, at the pres- 

 ent time, is understood in 

 few species of animals. 

 Boveri 1 and Wilson 2 find 

 in the sea-urchins that the 

 centrosome is the middle- 

 piece of the spermatozoon 

 and is exclusively of male 

 origin. Several other in- 

 vestigators have observed 

 in other animals its origin 

 from one germ-cell only, 

 usually the male, and it is 

 a question whether its male 

 origin may not be the com- 

 mon one. The significance 

 of this discovery and the 

 function of the aster will 

 be explained in the follow- 

 ing section. 



; Journal of Morphology, x., 1895. 



