364 RICHAED EVANS. 



Appendices : page 



A. Nutritive vacuoles and yolk bodies . . 422 



B. The origin of the cell groups found in types 



B and C . . . . 425 



C. Development and structure of spicules . 430 



D. Division of collar-cells and multiplication of 



chambers .... 432 



E. Mitotic division of tiie nuclei . . 434 



F. Technique ..... 436 



III. Ckitical Review of Previous Accounts and Conclusions . 443 



IV. TUEOKETICAL ....... 455 



Conclusion . . . . . . .461 



V. Bibliography ....... 462 



VI. Description op Plates ...... 463 



I. Introduction. 



This piece of work was begun about eighteen months ago 

 on the recommendation and under the superintendence of my 

 teachers, Professor E. Ray Lankester, M.A., F.R.S., and Mr. 

 E. A. Minchin, M.A., Fellow of Mertou College, Oxford. The 

 development of Spongilla was suggested to me as a subject 

 worthy of study, on account of the extreme confusion and 

 startling contradictions contained in the published accounts, 

 rendering it almost impossible to make out what was true or 

 Avhat was false. It was with the hope of determining the true 

 history and eliminating incorrect statements that I embarked 

 upon this study. 



In order to show what was the state of our knowledge, I 

 begin with a short historical account, whicli will be followed by 

 an abridged account of my own results. 



A. Historical Review. 



There is no need to go further back than the account given 

 by Ganin (4) in the year 1879. 



Ganin distinguished three layers of cells in the free-swim- 

 ming larva, which he called " ectoderm," " mesoderm," and 

 " entoderm " respectively. The " ectoderm " consisted of the 

 layer of flagellated cells at the surface, and the '' entoderm " 



