6 PROFESSOR ALLMAN. 



With regard to zooidal development as presented in the 

 well-known budding of Hydra, we find nothing of importance 

 added to the facts already known. But in the formation of 

 the sexual organs and in embryonal development we have a 

 scries of most careful researches, which have elicited many 

 unexpected facts and thrown new light on the entire subject. 



It has been long knoAvn that at certain seasons there are 

 found on the body of Hydra pustule-like swellings, which 

 were regarded by the earlier observers as the result of a 

 diseased condition of the animal. Ehrenberg was the first to 

 detect their true nature when he shoAved them to be sexual 

 organs — in some cases containing spermatozoa, in others ova ; 

 in other words, they are the testes and the ovaria of the 

 Hydra, and the mode of formation of these and of their 

 contents have been carefully followed by Kleinenberg. 



According to him the formation of the organ on which the 

 preparation of the spermatozoa devolves commences by a 

 more active growth of certain cells of that part of the 

 ectoderm Avhicli has been already described as the " inter- 

 stitial tissue." This change is limited to roundish, circum- 

 scribed spots ; the cells enlarge considerably, and assume the 

 form of polyhedral plates, their protoplasm becomes clearer, 

 and the spherical nucleus comes out distinctly. Then they re- 

 peatedly divide and pass into small, irregular, apparently 

 amoeboid cells, which become closely pressed together so as 

 to form a compact lenticular body. This is the testis. It 

 becomes gradually elevated into a conical projection, with its 

 summit produced into one or two papillae. In this state it is 

 invested by the other element of the ectoderm (nervo-muscular 

 tissue), whose cells have here become so much atrophied 

 that only a thin protoplasmal layer remains of them as an 

 external covering of the testis. 



In the mean time the nuclei of the testis cells break up into 

 numerous dark corpuscles, which soon disappear, and in their 

 place sharply-contoured, strongly refringent corpuscles make 

 their appearance. The cells then become converted into 

 little clear, spherical bodies, and it is out of each of these 

 that the spermatozoon is developed. On some one spot of the 

 surface of the sphere there is formed a fine process of proto- 

 plasm which soon exhibits active undulatory movements. 

 The time for the separation of the mature spermatozoon 

 from the mother cell has now arrived, the cilium is found 

 to be in union with one of the bright corpuscles in the 

 interior of the cell, and by its strong undulations, the cor- 

 puscle with its attached cilium is extricated as a mature 

 spermatozoon from the formative cellj which then becomes 



