24 EMBRYOLOGY 



ture, save that the parenchyma filling the internal cavity is of a different 

 histological character; it consists of a tissue comparable to hyaline 

 cartilage. The cleavage in this form is total and equal, although a 

 cceloblastula-stage is never developed (F. E. Schulze, I^os. 23 and 24). 



Chalinida. — The egg of Chalinula fertilis segments totally and un- 

 equally, according to C. Kellek (No. 9). The first act of cleavage 

 results in a larger and a smaller blastomere. The next stage which has 

 been observed shows three small and one large blastomere. By the 

 succeeding division of the small cells there results a stage in which six 

 small cells rest like a cap upon the large undivided blastomere, which 

 also soon divides. The small cleavage spheres are said to represent the 

 fundament of the ectoderm, the large ones that of entoderm and mesoderm 

 together. In the further course of cleavage, the small cells, having the 

 form of a shell, grow around the large ones in such a manner that there 

 results an epibolic gastrula, the mouth of which is entirely filled by the 

 solid cell-mass of the primary entoderm, which is plainly visible at this 

 point. The embryo now develops flagella on its entire surface, acquires 

 a more elongated form, and emerges as a planula larva. At its posterior 

 pole it presents a darker -coloured area of the superficial flagellate layer, 

 and this corresponds to the superficial entodermic portion. The earliest 

 spicules are very soon developed in the cells of the internal parenchyma. 

 The larva now attaches itself by its posterior pole, but very soon turns 

 over, so that it adheres to the support by the entire broad side of the 

 body. It now acquires the form of an irregular flat cake. The forma- 

 tion of the ampullae within is said to proceed in a different manner from 

 that described for Oscarella (p. 22) ; that is to say, individual entodermic 

 cells unite into compact groups, within which a cavity appears later. 

 The fundaments of the ciliated chambers (ampullas), which at first were 

 independent, then establish relations with a large central cavity arising 

 in the parenchyma, which soon breaks through to the outside at the apex 

 of the larva, thus producing the osculum. The larvae of most siliceous 

 sponges appear to belong to a type similar to that of the swarming larva 

 of Chalinula, thus the larvae of Esperia, Amorphina, Easpailia, and 

 Keniera (0. Schmidt, Metschnikoff), further those of Isodyctia and 

 Desmacidon, made known by Ch. Bareois. 



Reniera. — The larva of Keniera filigrana resembles the ones previously 

 described. It consists of a flagellate columnar epithelium and an in- 

 ternal cellular parenchyma. In the course of the further development 

 the layer of columnar cells ruptures at the anterior and posterior ends, 

 so that the internal parenchyma is exposed. The larva attaches itself 

 by the anterior pole, loses the covering of flagella belonging to the super- 

 ficial layer, and takes on the form of a flattened cake, while in the 

 internal parenchyma, the common fundament of entoderm and meso^ 

 derm, a cavity appears in the form of a fissure, about which the nearest 

 cells group themselves in the form of an epithelium. In this manner 

 the entodermic epithelium is separated from the mesoderm. The first 



