22 EMBRYOLOGY 



the apex of which the osculum (Os) breaks through (Fig. 6 D) 

 The inhalent pores (po) are formed as perforations at places 

 where the ampullae lie close to the ectoderm. The system 

 of inhalent canals, which arises later, must be referred to 

 invaginations of the ectoderm, that of the exhalent canals to 

 evaginations of the entoderm. The larva is not attached by 

 its entire base, but rests upon a few foot-like supports 

 (K. Heider, No. 8). 



The development of the Plakinid® appears to be closely related to that 

 of Oscarella. The swarming larva can on the whole be included in the 

 type just described. The metamorphosis into the attached stage has 

 not been accurately investigated ; it is certain, however, that here also 

 the ampullae arise as diverticula of a common central cavity (F. E. 

 Schulze). 



III.— Type of Development through a 

 Parenchymula Stage. 



The superficial layer of the swarming larva consists of a 

 cylinder epithelium, composed of long flagellate cells, and 

 encloses an internal space filled with embryonic connective 

 tissue. 



(a) The superficial layer presents on all sides cells of 

 nearly uniform condition. 



Ascetta. — By total and equal cleavage there is first produced a 

 coeloblastula-stage, which is similar to that of Oscarella. But even 

 before the hatching of the embryo, which attains an ovate form in 

 later stages, the immigration of cells into the internal cavity takes 

 place ; and this occurs at the posterior pole of the larva. In this 

 manner the primitive body cavity becomes filled with a connective-tissue 

 mesenchyma, the common fundament of the mesoderm and entoderm, 

 in which the permanent gastral cavity subsequently appears as a fissure. 

 The entoderm cells arrange themselves about this in the form of a 

 unilaminar epithelium (0. Schmidt, Metschnikoff). 



(6) The superficial layer in the region of the posterior 

 pole of the larva presents an altered condition of the cells. ^ 



1 [The investigations of Delage (No. I., Appendix to Literature) and 

 those of Maas (Nos. IV. and V.) on the development of the Cornacuspongia 

 (Ceratosa and Silicea) are of great importance. A satisfactory founda- 

 tion for the interpretation of the development of sponges in general has 



