144 PHYLUM CCELENTERA. 



nuclei) accumulate within the ovum as "yolk spherules" or "pseudo- 

 cells." With increase of size the ovum changes its form from 

 aiiKoboid to cake-like, and from that to spherical. Around the 

 spherical ovum a gelatinous sheath is formed. When the limit of 

 growth is reached, the nucleus or germinal vesicle divides twice in the 

 usual way, and two polar bodies are extruded at the distal pole. There- 

 after the ectoderm of the parent Hydra yields to the increasing strain 

 put upon it, and ruptures, allowing the ovum to protrude. By a broad 

 base it still remains, however, attached to the parent, and in this state 

 it is fertilised, the spermatozoon entering by the distal pole (Fig. 66, 4). 



The segmentation which follows is total and equal, and results in the 

 formation of a blastosphere (Fig. 66, 5). By inwandering, or by divi- 

 sion of the cells of the blastosphere, an internal endoderm is formed, 

 and this formation takes place on all sides. In a word, it is multipolar. 

 The segmentation cavity of the blastosphere is thus filled up, and tin- 

 two layers become differentiated from one another. 



The outer or ectodermic layer forms (a] an external " chitinoid " 

 shell of several layers ; (b] an internal membrane, homogeneous, thin, 

 and elastic ; and (c) the future ectoderm of the adult. In Hydra fusca 

 the egg is separated from the parent before the shell is formed, and is 

 fastened by its gelatinous sheath to aquatic plants ; in H. viridis and 

 H. grisea the egg falls off after the outer shell has been formed. In 

 all species the separation from the parent appears to be followed by a 

 period of quiescence lasting from one to two months. 



Within the shell differentiation at length recommences, but it pro- 

 ceeds slowly. Interstitial cells arise in the ectoderm ; a middle 

 lamella is formed ; a gastric cavity begins to appear in the midst of the 

 endoderm. Thereafter the shell bursts, and development proceeds 

 more rapidly. The embryo elongates, acquires a mouth by rupture at 

 the distal (sometimes called vegetative) pole. The inner sheath is also 

 lost, and the young Hydra fixes itself and begins to live as its parent 

 or parents did. 



Forms like Hydra. Even simpler than Hydra is Protohydra, 

 without tentacles, occurring both in the sea and in fresh water. An 

 American fresh-water form (Mtcrohydra jyderi) is known to liberate 

 free-swimming medusoids. This should be compared with the hydri- 

 form organism believed to be connected with the fresh-water Medusoid 

 I.imnocodhim found in the Victoria Regia tanks in the Botanic Gardens, 

 Regent's Park, London, and also in African lakes. A strange simple 

 polype Polypodium has been found as a parasite on the eggs of 

 sturgeons. Further details in regard to all these forms are much 

 wanted. 



Second Type ^/CCELENTERA. A Medusoid. 

 Class HYDROMEDUS^:. 



Hydra is too simple to be thoroughly typical of the 

 Hydromedusae. The class includes the hydroid colonies or 

 zoophytes, which may be compared to Hydra with many 



