266 GEO. W. TANNREUTHER. 



point where the sperm enters the egg (Brauer). The egg becomes 

 surrounded by a number of sperm, several of which may pass into 

 the egg membrane, but only one enters the egg. It is interesting 

 to note that the egg may remain susceptible to the sperm twenty- 

 four hours after it passes through the ectoderm, but if the sperm 

 are not added within twenty-four to thirty hours after maturation 

 or the passing of the egg to the exterior, fertilization will not 

 take place. 



Two lots of hydras with eggs, one twenty-four hours and the 

 other immediately after maturation, were placed in separate ves- 

 sels containing water and sperm added. Fertilization occurred 

 in both instances within two hours. The rate of cleavage was 

 similar in both cases. In the unfertilized eggs, the yolk spheres 

 cease dividing, gradually break down and the egg becomes a 

 hollow membranous sphere containing a fluid substance. 



Almost immediately after fertilization the peripheral cytoplasm 

 becomes free from yolk and is more finely granular than the 

 cytoplasm within the egg. 



The entrance of the sperm and union of male and female pro- 

 nuclei agree with Brauer's account. After the union of the pro- 

 nuclei the cleavage nucleus passes a short distance into the egg 

 from the animal pole and divides (PI. IX., Fig. 9). 



Cleavage. 



The cleavage of Hydra sp. ? is total, unequal and regular. 

 Brauer states that the cleavage is equal and total, but he gives no 

 figures to show the early cleavage stages. Kleinenberg says the 

 cleavage {H. viridis) takes place in a remarkable manner and that 

 pseudopodia or cleavage papillae are formed at the point where 

 the first cleavage begins. He also describes the second cleavage 

 as very erratic, and states that the egg undergoes peculiar changes 

 during cleavage. 



The cleavage of Hydra sp. ? does not exhibit such erratic con- 

 ditions as Kleinenberg describes for H. viridis. 



Just before the first cleavage the egg changes from a spherical 

 to a more oblong form, and the cleavage passes through the short 

 axis. This peculiarity is true only of the first, second and third 

 cleavages, being especially well marked in the first and second. 



