3l6 WILSON. [Vol. IX. 



8. The ectoderm begins to flatten from the posterior pole 

 forward during swimming life. 



9. The swimming larva attaches itself by the posterior pole, 

 but obliquely, so that it lies on its side. 



10. During attachment the entire ectoderm grows flat, and 

 afterwards spreads out round the sponge, as a membrane con- 

 taining no mesoderm. 



11. A peripheral mesodermic zone is formed, consisting of a 

 network of cells. To the fluctuations in the edge of this zone 

 are due the changes in contour of the young sponge. 



12. The canals and subdermal spaces arise as lacunae or 

 intercellular spaces in the parenchyma, which are at first inde- 

 pendent of one another, and only subsequently become con- 

 nected by the perforation of the intervening tissue. The 

 parenchyma cells immediately surrounding the lacuna develop 

 into a lining membrane of epithelioid cells. In their origin and 

 method of formation, there is no difference between subdermal 

 cavities, afferent canals, and efferent canals. 



13. Pores and oscula arise in the same way, as perforations 

 of the dermal membrane overlying the subdermal cavities and 

 efferent canals respectively. 



14. Flagellated chambers arise independently of each other 

 and of the canals, only later acquiring connection with the 

 canal system. A chamber may be formed from a group of 

 formative cells which arrange themselves in a hollow sphere, 

 the intercellular space becoming the cavity of the chamber. 

 Or else the chamber may be produced by the appearance of a 

 central cavity in a solid mass of fine cells, derived from the 

 division of formative cells. 



Previojis Kno'cvlcdge of tlic Development of Espcria {= Esper- 

 ella). — Besides the older observations of Metschnikoff (11), 

 Carter (2), and Oscar Schmidt (22) — see p. 370, section VI — 

 on the development of this genus, there are papers dealing 

 with the subject by Yves Delage (36, 1890), Maas (16, 1892), 

 and myself (35, 1891). 



