THE DEVELOPMENT OF HYDRA. 265 



remain attached to the egg by means of a cytoplasmic thread 

 and are found partly imbedded in the egg membrane beneath the 

 ectoderm surrounding the egg. When maturation is completed 

 an opening breaks through the ectoderm in the region of the 

 polar bodies. The egg with its contained yolk is very plastic, 

 and as the ectoderm contracts or is drawn back, the egg contents 

 is gradually forced through the small opening in the ectoderm. 

 It requires from one to three minutes for the contents of the egg 

 to pass through the opening of the ectoderm. The different 

 changes which the egg undergoes in this process are shown in 

 PI. X., Figs. 27—32. The egg now becomes situated in a basin- 

 like cavity of the ectoderm and is entirely free except for a small 

 portion at the vegetative pole (Figs. 3 1 and 32), where it is finally 

 attached to the cup-shaped ectoderm by means of transparent 

 pseudopodial processes of the egg membrane, which pass into the 

 ectoderm. An adhesive substance which is secreted by the 

 ectoderm also aids in their attachment. The egg membrane is 

 very tough and firm, and remains distinct during cleavage. 



The polar bodies after the egg passes to the exterior become free 

 in the water (PI. VIII., Fig. 1). They are more distinctly shown 

 in PI. IX., Fig. 8, pb, with their connecting thread of cytoplasm. 

 The chromosomes of the polar bodies do not become a homo- 

 geneous mass but retain their individuality. The first polar body 

 is larger than the second. The connecting thread of cytoplasm 

 becomes finer and longer, until it breaks loose from the egg and 

 leaves the polar bodies free in the water, where they almost im- 

 mediately go to pieces. Their connection with the egg may per- 

 sist until after the third cleavage, as shown in PI. XL, Figs. 36, 42. 



According to Brauer's account, the polar bodies disappear be- 

 fore cleavage begins. I was unable to distinguish any movement 

 of granular substance through the connecting thread between the 

 polar bodies and the egg, but, as the granular cytoplasm of the 

 strand is similar to that of the egg, it is highly probable that such 

 a flow of substance occurs. 



Fertilization. 

 Normally fertilization occurs within two hours after the egg 

 becomes free in the water. There is a small cavity formed at the 



