1 10 PART I. GENERAL ACCOUNT 



girdle of a single row of large cells is found near the posterior end. The ulti- 

 mate fate of these girdles has not been traced, but it is probable that they 

 later develop into the ciliated bands (pp. 112, 116). 



Almost the whole of the internal space of the embryo is occupied by a 

 dense mass of large cells of the archenteron, which are very rich in yolk and 

 oil. There is no sign that these cells are anywhere organized into any kind 

 of epithelium. At the front end of the archenteron active cell division has 

 produced the rudiments of the first coelom in the form of a large unpaired 

 pocket. It consists of large round cells, rich in yolk, and, at first sight, loosely 

 epithelial in arrangement (Fig. 16A). The pair of narrow lateral pouches, 

 which are the common rudiments of the second and third coeloms, appear at 

 about the same time (Fig. 16A). These grow from the lateral surfaces of the 

 archenteron on its anterior third, and it is possible that their origin is asso- 

 ciated from the first with that of the anterior coelom. At the posterior end the 

 tube-like lateral pockets are blind. The narrow lumen of each pouch can be 

 seen in transverse sections to lie between four or five cells (Fig. 76C). The 

 base of the pouch can be seen to merge imperceptibly on the medial side 

 into the cell mass of the archenteron, while on the lateral side it runs into the 

 walls of the first coelom. It is apparent that the initial formation of the lateral 

 pockets proceeds as a result of increasingly local multiplication of cells of the 

 archenteron or of the latero-posterior parts of the first coelomic pouch. The 

 lateral pouches then grow vigorously backwards. 



The complete separation of the coelomic pouches from the central cell 

 mass next takes place. The latter may now be regarded as a purely endodermal 

 gut primordium, though it must be emphasized once more that it shows no 

 trace of an enteric cavity. The pair of lateral coelomic sacs reach at this stage 

 almost to the hind end of the embryo (Fig. 11 A). 



Finally a deep annular wasp-waist appears in the hind part of the embryo 

 dividing off a small posterior region of the body which represents the trunk 

 segment or metasoma (Fig. 11 B). The endoderm in this hind segment, which 

 is proper to the metasoma, remains connected by a narrow stalk to the rest 

 of this tissue, but the hind ends of the coelomic sacs are pinched off and their 

 cavities become the third coelomic cavities or metacoeles (Fig. 11 B). At the 

 same time a broad ciliated ring or girdle, furnished very densely with short 

 cilia, is differentiated near the front end of the embryo. 



The coelom of Pogonophora thus arises by enterocoely, as is general in 

 Deuterostomia. The anterior part of the archenteron forms an unpaired 

 pouch — the first coelom — and a pair of lateral pouches originate in conjunc- 

 tion with this to give rise ultimately to the second and third coelomic cavities. 



