8 PROFESSOR ALLMAN. 



to the other. From this mass the ova or spermatic cells 

 become differentiated; it increases rapidly in volume, and this 

 increase appears to take place from additions at the side which 

 is in contact with the endoderm, thus leading to the conclu- 

 sion that additions are made to it by continuous transforma- 

 tion of the endodermal tissue. It is at all events certain 

 that the generative elements are developed from it in a cen- 

 trifugal direction, so that the more mature ova or spermatozoa 

 are always foupd towards its ectodermal surface, and the 

 nearer we approach the endoderm the more immature does 

 the entire tissue appear — a state of things which though not 

 absolutely conclusive as to the endodermal origin of the gene- 

 rative tissue, is most easily explained by regarding this tissue 

 as so formed. 



Another fact in favour of the same view is the occurrence 

 of a delicate membrane which, in some cases, may be seen 

 lying upon the outside of the mass of the ova and separating 

 this from the surrounding ectoderm. This membrane I believe 

 to be the " muscle lamella " of Kleinenberg much atrophied 

 by the pressure of the included mass, and thus reduced to an 

 extremely thin pellicle without any trace of fibrillse. If so, it 

 is impossible that the ova could have had their origin in the 

 tissue of the ectoderm. 



A still further argument in favour of the endodermal origin 

 of the generative elements may be derived from the very ex- 

 ceptional condition presented by a few hydroids, and notably 

 by Sertularia pumila. In this species ova are produced in the 

 usual way within a sporosac. But besides occurring in the 

 sporosac, they are also found in the walls of the blastostyle 

 or columnar zooid from which the sporosac springs as a bud. 

 These blastostylic ova, though probably originating, as in 

 other cases, between the endoderm and ectoderm, are here 

 deeply imbedded in the endoderm so deeply that they project 

 into the cavity of the blastostyle, from which they are sepa- 

 rated only by a very thin layer of the endoderm. 



On the whole, the evidence, so far as the marine hydroids 

 are concerned, appears in favour of the endodermal rather 

 than the ectodermal origin of the generative elements. Klein- 

 enberg, however, is very positive that in Hydra these elements 

 are derived from the interstitial cells of the ectoderm. With 

 his very careful observations and unexceptionable manipula- 

 tions it is scarcely possible to believe that he has been led into 

 error, and yet it would be a curious anomaly to find that so 

 fundamental a difference in this respect lay between the fresh- 

 water hydra and its marine representatives. 



The formation of the egg is next described, and here we 



