342 THE AMERICAN MONTHLY [Dec 



(5). A little later and the granular cells become push- 

 ed in towards the columnar cells. This is probably due 

 to the position in which the embryo is lying — the granu- 

 lar cells being bounded by spicules whilst the columnar 

 cells are free to expand. This is known as the pseudo- 

 gastrula stage. (A. 6, 7.) 



(6). About this time the embryo bursts through the 

 wall of the flagellate chamber ; thegranular cells are push- 

 ed out again and the embryo has the form of a hollow 

 sphere (see B. and B. 1). The columnar cells now pro- 

 duce cilia and the embryo, by their means, is free to move 

 in the water. This is known as the amphiblastula stage. 

 (B. 2.) 



(7). True invagination now takes place, the columnar 

 cells being pushed in whilst the granular cells are arrang- 

 ed in a single layer outside and the segmentation cavity 

 reduced to a mere slit. This is the gastrula stage. (B. 3.) 



(8). A little later and the embryo attaches itself by 

 its open end to some foreign object. The outer granular 

 cells become flattened, the columnar cells lose their cilia 

 and a gelatinous layer is produced between the other lay- 

 ers. Three distinct layers are, therefore, now present — 

 ectoderm and endoderm with a layer of mesoderm be- 

 tween. (B. 4.) 



(9). The next stage consists in the elongation of the 

 embryo into a tubular form when it becomes flattened at 

 the top and perforated by an osculum. The walls are 

 pierced by pores, spicules appear in the gelatinous layer 

 and the ciliated cells developed into collared cells. This 

 is known as the olynthus stage. (B. 5, 6.) 



(10). Special chambers are gradually budded out from 

 the gastral cavity and the collared cells become restrict- 

 ed to these chambers, the gastral cavity being lined by a 

 layer of flattened ectodermal cells. 



From the Victoria Naturalist, August, 1900. 



