20 EMBRYOLOGY 



The ami^hiblastula-stage seems to occur in the hfe-history of many 

 Calcarea. It has also been found in Ascandra contorta (Baekois), in 

 Ascanclra Lieberkilhnii (Keller), and Leucandra aspera (Keller, 

 Metschnikoff). The genus Ascetta develops according to another type. 



II.— Type of Development through a Swarming 

 Cceloblastu la-stage. 



The egg of Oscar ella lohuJaris (Halisarca lohularis) de- 

 velops in the trabeculfe of the tissue in the internal parts of 

 the parent, which are -without flagellate ampullge, and under- 

 goes total and equal cleavage, by which there are formed at 

 first two, then four, eight, sixteen, etc., blastomeres of equal 

 size. At the sixteen-cell stage a distinct cleavage cavity can 

 be recognized within. By further cell proliferation there is 

 formed a hollow sphere (coeloblastula or archihlastula) , the 

 wall of which is composed exclusively of cubical cells of equal 

 size arranged in a single layer (Carter, No. 3 ; Barrois, 

 No. 2 ; F. E. Schulze, No. 20). 



Shortly before swarming, the elements of the body-wall 

 lengthen out into columnar epithelial cells, each of which 

 acquires a flagellum at its outer end. The swarming larva 

 (Fig. 6 A) possesses an approximately ovate form, and ex- 

 hibits a blunt yellowish pole, which is directed forwards in 

 swimming, and a posterior, more pointed brownish-red pole. 

 The wall of the blastula consists of a single layer of 

 cylindrical flagellate cells. The internal cavity contains no 

 cells, and is filled with an albuminous fluid (F. E. Schulze). 



By the invagination of one pole of the larva this stage 

 passes into a hemispherical gastrula-stage, which, like that 

 of Sycandra, now attaches itself by its gastrula mouth to a 

 support (Fig. 6jB). Thus there arises a shallow, cap-shaped 

 larva, the wall of which consists of two layers (ectoderm 

 and entoderm) and the inner cavity of which must be con- 

 sidered as the archenteron. The gradual closure of the 

 wide gastrula mouth now follows ; and, at the same time, by 

 a complicated process of folding, the first flagellate ampulla? 

 arise as diverticulaB of the archenteron (Fig. 6C). The 

 mesodermal connective-tissue layer originates by the mi- 

 gi'ation of cells into the space embraced between the 



