MUSEUM OF COMPARATIVE ZOOLOGY. 587 



author has attached far too little importance to his observations on Sagitta 

 (PL X. Fig. 6). He remarks that the attraction is therefore exercised not so 

 much by a simple spermatozoon as by a fusion of this with the sarcode, and 

 that it is this union which gives rise to the male pronucleus. This statement 

 approaches so closely the view I have already advocated, that I should be in- 

 clined to think our ideas on this point identical, were it not that he sub- 

 sequently explains his position in a manner which shows clearly that in his 

 opinion it is the substance of the male pronucleus which exercises the attractive 

 influence rather than a force liberated in the act of the union. He says, sub- 

 stantially : " The male center is surrounded soon after its formation by a star 

 of unipolar rays. Shortly the centre, represented by the body of the more or 

 less modified spermatozoon, is surrounded with clear protoplasm. This mass 

 continues to increase, — a fact which seems to indicate that the sarcodic rays 

 are the expression of centripetal currents of protoplasm coming from the vitel- 

 lus. However that may be, it is certain that the aster is formed around a 

 modified spermatozoon which is found at its centre ; that it is a result of the 

 action exercised hy this corpuscle upon the surrounding vitellus. If this is so, it 

 should be explained why the evidence of this attraction ceases when the 

 nucleus has attained its maximum size. 



The phenomena of attraction, Fol continues, are perhaps less striking for 

 the female than for the male pronucleus, but they exist none the less. They 

 are, — (1.) radial lines, which continue to augment in proportion as the pro- 

 nucleus absorbs vitelline sarcode, and are only effaced at the moment when it 

 has come to rest ; (2.) the centripetal advance of sarcodic currents, of which the 

 radial striae are the visible expression, and the direction of which is indicated 

 by the growth of the nucleus ; (3.) the displacement of the pronucleus itself 

 from the periphery toward the centre of the yolk. 



The centres of attraction which appear at the poles of the amphiaster of 

 segmentation are due to a fusion (rencontre et alliage) of nuclear substance 

 with vitelline sarcode at the circumscribed points (poles) where the contour 

 of the elongated nucleus becomes lost. But this is not the first process pre- 

 liminary to the formation of the amphiaster. On the contrary, in the case 

 of the sea-urchin the appearance of a mass of sarcode around the nucleus, 

 as well as the " pinnate figure," precedes. The latter appears to be the 

 expression of centrifugal rather than centripetal currents ; the formation of 

 typical asters, on the other hand, only dates from the moment when the nuclear 

 and vitelline substances enter into communication at the poles of the nucleus. 

 The three cases have in common this point : that the phenomena of attraction 

 (and of repulsion) may precede the mingling of two diverse substances, but that 

 they attain their full development, and interpret themselves by the formation 

 of a veritable aster, only when there has been a fusion of the two substances ; 

 the point of fusion is then always the centre of the system of rays. It seems 

 to me this last statement is more in harmony with the view I have maintained 

 than it is with the ideas which Fol has himself previously advocated. In my 

 opinion it is the point of fusion, and not necessarily the product of the fusion, 



