FREE-SWIMMING SPOROSACS OF THE HYDROID GENUS DICORYNE. 265 



living specimens in captivity were not sufficient to establish this point with 

 certainty. 



The subsequent life of the sporosac is probably of short duration. The ectoderm 

 is ruptured (fig. 6), the oocyte escapes, and maturation and fertilisation no doubt 

 soon take place. We have, however, seen three examples in which fertilisation and 

 egg-cleavage had occurred while the egg was still contained in the remains of the 

 sporosac, but these are probably exceptional cases and may have been determined by 

 the abnormal conditions of captivity. All the other developing eggs were found 

 free from the sporosacs. Whether the eggs, when liberated, float or sink to the 

 bottom we are unable to say. 



The blastostyles apparently produce successive series of sporosacs, for on a blasto- 

 style which bears almost fully formed sporosacs are oocytes only about 15/x in 

 diameter in small elevations of the ectoderm, forming incipient sporosacs of a 

 new series. 



B. The Male Sporosacs (PI. VIII, figs. 8, 9, 10). 



The material at our disposal for the study of the male sporosacs, while not so 

 complete as for the females, is sufficient to enable us to give an account of the main 

 features of their development. Several blastostyles bearing male sporosacs in their 

 early and middle stages of development were present on three of the colonies ; these 

 have been studied entire and in longitudinal serial sections, and about a score of 

 free male sporosacs have also been examined. 



The male sporosacs are confined to a narrow zone about the middle of the 

 blastostyle. The first indication of their appearance is the differentiation in the 

 base of the ectoderm of one or more cells (usually there seem to be two or three). 

 These are ovoid in form, their protoplasm stains more deeply than that of the 

 neighbouring cells, and their nuclei are larger and of a more vesicular type, with 

 peripheral granules of chromatin and a large nucleolus (fig. 8). 



The elevation which eventually becomes the sporosac seems to be due to the 

 almost simultaneous activity of both cell-layers. The ordinary ectoderm cells of 

 such an incipient sporosac multiply pari passu with the spermatogonia, and the 

 subjacent endoderm, judging from the special character of the cells, is also in a state 

 of considerable activity, so that the condition seen in fig. 9 soon results. 



The thick ectoderm of the distal end of the sporosac is composed chiefly of 

 a mass of deeply staining cells — the spermatogonia — on the periphery of which 

 are scattered cells which stain more faintly, these being the ordinary cells which 

 will later form the ectodermic envelope of the sperm mass. The endoderm cells 

 below the spermatogonia are more granular and stain rather more deeply than the 

 cells proximal to them. This endodermal spadix is either solid or has a narrow 

 central lumen continuous with the ccelenteron of the blastostyle. The mesogloea of 

 the sporosac is thin and grades into that of the blastostyle. Such a sporosac, having 



TRANS. ROY. SOC. EDIN., VOL. LI, PART I (NO. 6). 38 



