No. 2.] THE EGG OF BUFO LENTIGINOSUS. 327 



radial cytoplasmic systems are formed which become the poles 

 of the segmentation spindle. Whatever its origin, and I am in- 

 clined to agree with those who consider the astrosphere to be 

 purely cytoplasmic, the astrosphere is present through all 

 stages of fertilization, and apparently plays an important role 

 in the various processes leading to the division of the cell. 



The entire spermatozo6n*enters the &%g. The tail (PI. XXX, 

 Fig. 45) is found for a short time and is then absorbed, as in 

 Axolotl. I have been unable to follow the history of the mid- 

 dle-piece after it enters the eggs, as it stains very faintly ; but 

 I believe it is absorbed with the tail and takes no part in the 

 fertilization processes. 



Hill ('96), Kostanecki and Wierzejski ('96), Wilson ('95), and 

 others have described the rotation of the sperm-head through 

 an angle of 180° after it enters the egg. This is done, appar- 

 ently, in order that the middle-piece from which the astro- 

 sphere arises, according to these investigators, may precede the 

 male pronucleus in its migration towards the female pronucleus. 

 I have found no indication of a rotation of the sperm-head in 

 the Q.%g of Bufo. 



The path traced by the entering spermatozoon is similar to 

 that described by Roux for the frog's Q.g%. The sperm moves 

 at first centripetally, describing the "entrance path," and later 

 curves more or less abruptly towards the female pronucleus, 

 thus describing the " copulation path " (PI. XXX, Fig. 46). 



In all pigmented amphibian eggs the sperm-path is marked 

 from its beginning by a well-defined trail of pigment granules. 

 Many investigators agree with Hertwig : "Dass von der pig- 

 mentirten Rindenschicht ein vom Kern ausgezogener Theil 

 sich abschniirt und mit nach dem Centrum wandert ; hierbei 

 losen sich Pigmentkorperchen von Stelle zu Stelle ab und las- 

 sen so noch spater die Strasse erkennen, auf der die Einwan- 

 derung des Spermakerns erfolgt." Pick's objections to this 

 explanation of the origin of the pigment trail seem well 

 founded. If the sperm-head can penetrate the thick egg-mem- 

 brane, it is inconceivable that it cannot also bore its way 

 through the pigment layer below. Moreover, the sperm-path 

 is so deeply pigmented that it does not seem possible that so 



