EARLY DEVELOPMENT OF HYDRA, 23 



not impossible, however, that the healthy and vigorous egg may- 

 be sometimes thus infested. 



Egg Membrane. 

 For some time before breaking through the ectoderm the sec- 

 tions show an egg membrane of varying thickness to be present. 

 This is sometimes difficult to find until shortly before the emer- 

 gence of the egg. When the egg becomes surrounded by water 

 the membrane is invisible owing to its transparency, but quickly 

 becomes evident upon the application of killing fluids. 



Cleavage. 



The first cleavage is total and equal. It begins at the distal 

 pole of the egg and in a plane usually at right angles to the 

 long axis of the body. Papillae are well developed at the point 

 where the cleavage begins and are easily seen on either side of 

 the cleavage furrow when very young (Fig. 23, a, PI. III.). 

 This agrees with Kleinenberg's observations on the cleavage of 

 H. viridis. The furrow deepens and its edges close over as it 

 advances, its position being shown by an opening extending 

 completely through the egg (Fig. 23, b and c, PI. III.). The first 

 cleavage is accomplished in about thirty minutes. 



The second cleavage takes place in a plane at right angles to 

 that of the first and is also total and equal. The papillary proc- 

 esses are also to be seen in the cleavage furrow. The four 

 resulting blastomeres are of nearly equal size (Fig. 23, d, PI. 

 III.). The third cleavage takes place in a plane at right angles 

 to the other two or parallel with the long axis of the body. In 

 this case the cleavage furrow is much less distinct and is formed 

 more slowly in some of the blastomeres than in others, with the 

 result that from this point onward the blastomeres vary in size 

 and give rise, to an extremely irregular mass of cells (Fig. 23, 

 e, f, g and h, PI. III.). In the third and even later cleavages 

 papillary prominences are frequently to be seen at the beginning 

 of the cleavage furrow. These conditions greatly resemble those 

 described by Kleinenberg for H. viridis. The blastomeres vary 

 considerably in size, those at the proximal pole usually being of 

 somewhat greater dimensions. At the conclusion of the third 



