THE CCELOMATA INVERTEBRATA 193 



the production of two internally situated rows of cells, the mesoderm 

 bands, passing forwards from the posteriorly situated original pole 

 cells. This process goes on for a long time, and in its early stages 

 considerably reduces the blastocoel cavity. Thus we have a hollow 

 embryo with an upper layer of small cells, a lower layer of large cells, 

 and intermediate lines of cells, the primary mesoderm. 



After this, gastritlation, the conversion of the blastula into a 

 gastrula by the formation of the entoderm and enteron, takes place, 

 The lower larger cells invaginate, causing the edges of the cake tc 

 turn in and at the same time the upper cells gradully grow over more 

 and more. Thus is formed an oval still somewhat flattened structure 

 covered with smaller cells, the ectoderm, and having on the ventral 

 surface a large oval aperture, the blastopore, which leads into the 

 archenteron or primitive gut, whose walls are formed of the large 

 cells, the entoderm. In this way we have laid down the three layers 

 characteristic of the adult ccelomate, the ectoderm, entoderm, and 

 mesoderm. 



The embryo grows slowly in length and simultaneously the blasto- 

 pore gets narrower, ultimately closing up from the posterior end 

 forward, leaving only a tiny round opening at the front, which persists 

 and marks the place where the ectoderm will grow inwards to form 

 the stomodoeum. As soon as this last structure has made its appear- 

 ance the embryo is able to swallow the nutritive fluid contained in 

 the cocoon, and this causes the archenteron to swell until it touches 

 the ectoderm save in the regions occupied by the mesodermal bands, 

 thus obliterating the blastoccel. 



With further growth noticeable changes take place in these rows 

 of cells, which with the closure of the blastopore come to lie quite 

 near together close to the mid- ventral line. Not only do these bands 

 increase in length by transverse divisions, but at the front end they 

 also become wider, and instead of consisting of only one cell they are 

 transformed by longitudinal divisions firstly into bands of two or 

 three cells deep. Subsequently they become wider and wider, and 

 at the same time each band is cut up at its anterior end into a series 

 of masses which form a series of paired blocks on each side of the 

 middle line, the mesodermal somites. They are at first small and solid, 

 but a cavity, the ccelom, appears in them, and they increase fairly 

 rapidly in size and grow upwards between the ectoderm and entoderm 

 so as to surround the gut. Finally, they meet dorsally to the archen- 

 teron, and then their cavities run into one another above and below, 

 so that the mesoderm is divided into two parts, one surrounding the 

 gut and the other lining the ectoderm. In this way we have estab- 

 lished a body wall composed of ectoderm and a layer of mesoderm, 

 constituting the somatopleure, and a gut, wall or splanchnopleure, 



