EMBRYOLOGY OF CRYPTOBRANCHUS 143 



are here finer than elsewhere at the same level. The crescent 

 has become larger, and thicker at the ends than in the middle. 

 The head of the spermatozoon is shortened to the form of a thick 

 spindle and stains deeply; the tail persists in an abbreviated con- 

 dition. In the case of the spermatozoon shown in figure 44, an 

 aster is found at a slightly higher level than the sperm head and a 

 little nearer to the egg-nucleus. A study of the protoplasmic paths 

 of the aster and the sperm-head shows that they have separated 

 at a point midway in the path of the latter. 



Ten and one-half hours after fertilization (see figs. 45 and 46) 

 the spermatozoon has become transformed into the sperm- 

 nucleus, which is amoeboid in form; the tail of the spermatozoon is 

 represented only by fragments. At this time the sperm-nucleus 

 lies about half as far from the surface as the copulation-nucleus 

 shown in fig. 52. Immediately beneath the sperm-nucleus lies 

 a considerable mass of cytoplasm, perhaps formed at the expense 

 of the crescent which is dwindling except at the extreme ends. 

 The remains of the crescent, and the characteristic appearance of 

 the surrounding yolk, enable one readily to distinguish the sperm- 

 nucleus from the egg-nucleus. The sperm-nucleus is smaller 

 than the egg-nucleus, and like the latter does not stain deeply at 

 this time. 



The cytoplasmic changes in the egg caused by the invasion of 

 the spermatozoon may be tentatively interpreted as follows: 

 Under the influence of the centrosome, whose seat appears in this 

 case to be in the middle-piece, egg-cytoplasm collects about the 

 neck of the spermatozoon. Here the centrosphere and event- 

 ually the entire aster is formed. As the spermatozoon invades the 

 deeper region of coarser yolk particles, the resistance offered to the 

 progress of the accompanying mass of cytoplasm causes it to 

 flatten out like a bullet fired against a wall, assuming a form cres- 

 cent-shaped in section. Presently the spermatozoon, during its 

 transformation into the sperm-nucleus, comes almost to a full 

 stop, allowing the mass of cytoplasm again to assume a spherical 

 form. 



Numerous observers have described in both invertebrates and 

 vertebrates a rotation of the sperm head after it enters the egg, 



