THE DEVELOPMENT OF HYDRA. 267 



The first cleavage begins at the animal pole. The eggs of 

 Hydra sp. ? do not show any blunt projections or cleavage 

 papillse, as Kleinenberg and Andrews describe in Hydra viridis. 

 As the first cleavage furrow deepens, pseudopodia are formed, 

 which project into the cleavage furrow (PI. XL, Fig. 34). These 

 pseudopodial projections change in shape by contraction or expan- 

 sion as the cleavage furrow progresses from one side to the other. 

 The pseudopodia of one side may fuse with those of the opposite. 

 When this occurs, they do not pull apart but remain connected 

 during cleavage. Those that do not fuse are soon drawn in. 

 The living material shows a movement of the granular cytoplasm 

 containing a few yolk spheres from one blastomere to another 

 through these connections. New pseudopodia continue to form 

 until the cleavage furrow reaches the opposite side. When the 

 cleavage is nearly complete the furrow closes at the point where 

 it first began (Fig. 35), and when completely closed the pseudo- 

 podial connections are no longer visible in the living egg. The 

 bottom of the cleavage furrow shows a distinct opening which 

 extends entirely through the egg laterally and progresses with 

 the cleavage from the animal to the vegetative pole (Fig. 35). 

 This interesting phenomenon was observed by Kleinenberg and 

 Andrews in H viridis, especially during the first cleavage. Fig. 

 36 represents the first cleavage completed and the relation of 

 the egg to the ectoderm as it appears in the living hydra. Sec- 

 tions of the different stages of the first cleavage are shown in 

 PI. IX., Figs. 9-1 1. After the first cleavage is completed, the 

 nuclei divide before the second cleavage begins (Fig. 11). 

 Brauer states that the second cleavage begins before the first is 

 completed. 



The second cleavage passes through a plane at right angles to 

 the first and is nearly equal. This cleavage is similar to the first, 

 as shown in PI. XL, Figs. 37 and 38. A polar view of the second 

 cleavage is shown in Fig. 39. A section of the four-cell stage 

 taken at right angles to the polar axis (PI. IX., Fig. 12) shows a 

 number of pseudopodial connections or bridges persisting. The 

 third cleavage differs from the first and second in that no pseudo- 

 podia are formed in the cleavage furrow. PI. XL, Figs. 40-42 

 represent the different stages of the third cleavage as it appears 



