O-i Proceedings of the Royal Societij of Victoria. 



vertical, and result in tlie formation of altogether eight 

 pyramidal segments, arranged in a single layer, with a small 

 cavity in the centre, the embryo at this stage being cushion- 

 shaped. Each segment now divides horizontally, so that we 

 have an embryo composed of two layers of eight cells each. 

 Segmentation goes on until the embryo has the form of 

 a hollow sphere — the Blastosphere — whose wall is composed 

 of a single layer of cells, eight of which, situate at one pole 

 of the sphere, are distinguished from the remainder by their 

 granular appearance. These eight cells increase to about 

 thirty-two in number, and become pushed in or invaginated, 

 still, however, remaining as a single layer, so as almost to 

 obliterate the cavity of the blastosphere (segmentation 

 cavity). The remaining cells of tlie blastosphere become 

 much elongated and ciliated. The embryo is still enclosed 

 within the parental tissues. To this stage — characterized 

 by the invagination of the granular cells — the name 

 Fseudogastrula has been given ; according to Balfour, no 

 importance can be attached to it. The embryo now soon 

 leaves the parent, and by the time this takes place the 

 granular cells have inci'eased in bulk and become completely 

 everted again, still remaining as a single layer. 



The free swimming embryo (or larva), known as an AonpJd- 

 blastula, is oval or egg-shaped, and transversely divided 

 into two halves — a front half, composed of a layer of very 

 numerous, elongated, ciliated cells, and a hinder half 

 composed of the layer of granular cells, now thirty-two in 

 number. Some fifteen or sixteen of the granular cells, viz., 

 those which toucli the ciliated cells, form a special ring. 

 Balfour states that " during the later periods of the amphi- 

 blastula stage a cavity appears in the granular cells dividing 

 tbem into two layers." This statement appears to be based 

 upon Metschnikoff's observations, to which I shall have to 

 refer presently. 



After swimming about for some time, the ciliated half of 

 the larva becomes invaginated into the granular half, 

 obliterating the segmentation cavity and giving rise to the 

 Gastrula stage. " The two layers of the gastrula," says 

 Balfour, " may now be spoken ol as epiblast and hy])oblast." 

 The gastrula next becomes attached to some object b}- its 

 mouth, the attachment being effected by the granular 

 (epiblast) cells of the special ring alreadj^ referred to. 

 "Between the epiblast cells and the hypoblast cells which line 

 the gastrula cavity there arises a hyaline structureless 



