ANATOMY AND DEVELOPMENT OF HYDRA. 17 



walls of the sac-like body thus formed, and not, as main- 

 tained by Kowalewsky and Semper, in nearly allied 

 animals, by an invagination of the walls. 



Another question of great importance in this inquiry is 

 how far the origin of the various tissues and organs in the 

 respective* germ - lamellae corresponds through the several 

 groups of the animal kingdom. We know that in the 

 Vertebrata there are formed from the outer lamella, the great 

 nerve centres and organs of sense, and the epidermal struc- 

 tures ; from the middle lamella the muscular, skeletal, and 

 vascular systems ; and from the inner lamella the lining of 

 the digestive system and the glands connected with it. 

 Opinions vary as to the original seat of the generative 

 system. 



Now, accepting the view that the ectoderm of the CcBlen- 

 terata represents the united outer and middle germ-lamellae 

 of the higher animals, and accepting also the hypothesis 

 that the superficial cells of the ectoderm in Hydra with their 

 processes represent a nervo-muscular system, we shall find a 

 remarkable correspondence as to the origin of the specialised 

 tissues between the Hydroida and the Vertebrata. In the 

 Hydroida the tissues are few, but these few are similar in their 

 origin from the primitive embryo layers to the equivalent 

 tissues of the higher; for the nervo-muscular tissue of the 

 Hydroida has its foundation in the ectoderm, which is equiva- 

 lent to the united outer and middle gerin-lamella, while the 

 digestive surface is plainly formed by the endoderm or inner 

 germ-lamella. Here as in the higher animals, the origin of 

 the generative system is still an open question, and it is 

 probable that it is not in every case derived from one and the 

 same lamella, for while Kleinenberg is very certain of having 

 traced it to the ectoderm in Hydra, my own researches are in 

 favour of its endodermal origin in other Hydroida. 



A still further interesting point of identity follows from 

 Kleinenberg's discovery of the origin of the germ shell in 

 Hydra. This chitinous investment he shows to be a true 

 epidermal structure formed by the entire conversion of the 

 most superficial cells of the germ at a very early period in 

 the development ; and then entirely cast off after remaining 

 for a time as a transitory embryonal organ. 



We have thus exactly, as in the Vertebrata, the most 

 superficial portion of the outer germ-lamella developing an 

 epidermis whose very early appearance and transitory, nature 

 are the only important points in which its history differs 

 from that of the epidermal layer in the Vertebrata. 



It is true that nothing of this kind can be detected in the 



VOL. XIV. NEW SER. B 



