PKELIMINARV GENERAL SKETCH 



187 



aster or amphiaster) is developed in the neighbourhood of the sperm- 

 nucleus, and this in a large number of cases gives rise or is definitely 

 related to the cleavage- 

 amphiaster (coelente- 

 rates, flat-worms, echi- 

 noderms, nematodes, 

 annelids, arthropods, 

 mollusks, tunicates, ver- 

 tebrates). In many of 

 these cases the sperm- 

 aster, which by divi- 

 sion gives rise to the 

 amphiaster, has been 

 found to arise in inti- 

 mate relation with the 

 middle - piece of the 

 spermatozoon ; e.g. in 

 echinoderms(Flemming, 

 Hertwig, Boveri, Wil- 

 son, Mathews, Hill, etc.), 

 in the axolotl(Fick) and 

 salamander (Michaelis), 

 in the tunicates (Hill), 

 annelids (Foot, Vejdov- 

 sky), insects (Henking), 

 nematodes (Meyer, Er- 

 langer), and mollusks 

 (Henking, Kostanecki, 

 and Wierzejski). The 

 agreement between 

 forms so diverse is very 

 strong evidence that this 

 is a very general phe- 

 nomenon, and it is one of 

 great interest, owing to 

 the fact that the middle- 

 piece is itself derived 

 from or contains the 

 centrosome of the sper- 

 matid.^ 



The facts may be il- 

 lustrated by a brief 

 description of the phe- 



Fig. 94. — Entiance and rotation of the sperm-head and 

 formation of the sperm-aster in the sea-urchin, Toxopneustes 

 {A-F. X 1600 ; G, //, X 800). 



A. Sperm-head before entrance ; n. nucleus ; m. mid- 

 dle-piece and part of the flagellum. B. C. Immediately 

 after entrance, showing entrance-cone. D. Rotation of ihe 

 sperm-head, formation of the sperm-aster about the middle- 

 piece. E. Casting off of middle-piece; centrosome at focus 

 of the rays {cf. Fig. 12). The changes figured occupy about 

 eight minutes. F. G. Approach of the germ-nuclei ; growth 

 of the aster. 



1 Cf. p. 170. 



