1893.] MlCKOSCOriCAL -lOCRNAL. 67 



the same way. This is termed " asexual development." Any 

 part of Hydra has the power, if severed, by accident, from the body, 

 of reprodncinji; all the rest of the body, and thus the more you 

 lacerate and cut up such a body the more Hydras you produce. 

 Hut, besides these asexual modes of reproduction. Hydra has the 

 power, resident in the gonads, of producing a wholly new 

 bodv from the combined product of a single ovum and a single 

 spermatozoon ; this process is called " sexual reproduction." its 

 steps are as follows : The ovary is composed of angular cells, 

 primitive ova ; these are amo'boid w ithin the ovary, and they 

 struggle tor life, the stronger devouring the weaker, till finally 

 one large egg-cell, ovum, results (tig. 6). which consists of a 

 nucleus and protoplasm, and a large amount of food-yolk, called 

 deutcrop/as/n. which is of a fatty nature and is destined to 

 serve as food for the growing ovum. The cells of the male 

 gonad, or spcrmary^ take an opposite course, dividing and redivid- 

 ing to form mother cells of spermatozoa (see fig. 7), which 

 become at first ama-boid, then globular, then thrust out a long 

 Hagellum. and finally shrink to a minute head on the end of this 

 flagellum. Myriads of immensely active spermatozoa striking 

 their tails about burst from the spermary and, moving 

 vaguely, swim through tlie water, and some find the ovum in the 

 female gonad, or ovary. The ovum imdergoes its earlier changes 

 in the ovarv. Before the advent of the spermatozoon, it has 

 assumed a spherical shape, the nucleus has travelled to the 

 margin, and some of its substance passed out of the wall of the 

 ovum in the form of two minute droplets — polar globules. 

 The spermatozoon enters and a second nucleus forms about it, 

 and the two nuclei then fuse to form a single completed nucleus, 

 and this act is called the fertilization of the ^%^- It is necessary 

 to the future development of the egg, for otherwise it does not 

 segment ; the formation of polar globules is regarded as a sort of 

 feeble attempt to segment. 



Immediately after fertilization the ovum segfueiits or rlivides 

 into two cells in a manner precisely like the division of a proto- 

 zoon, but the cells remain connected ; it does not stop here, but 

 presently each half divides into quarters, and these into eighths. 

 The cells keep on dividing, and form a mass of 16 cells, 32 cells, 

 64 cells (see figs. S. 9, 10, 11, 12), until finally they have grown 

 to the form of a hollow sphere of many cells, but only one cell 

 thick, called a 7norula^ the cavity within being called the seg- 

 mentation cavity * 



This segmentative activity pushes the sphere out to a certain 

 size, after which new cells as they are formed are pushed in, or 

 push others into the segmentation cavity until a .solid niorula is 

 formed. The cells thus pushed in are the endoderm, and those 

 remaining on the surface of the sphere are the ectoderm. The 



* Brauer, Zeits. f. u. Zool., vol. 52, page 169, 1891. 



