PORIFERA 



17 



size and hj their granular plasma. With the closure of this 

 lower opening the embryo becomes a spherical blastosphere. 

 The granular cells now enlarge and multiply to the num- 

 ber of about thirty-two ; the other cells meanwhile increase in 

 numbers, and lengthen out into tall columnar prisms (Fig*. 2 

 M), each of which develops a flagellum at the surface. The 

 large, richly granular cells now fold into the segmentation 

 cavity, and the last stage to be passed in the body of the parent, 

 the so-called pseudo-gastrula stage, is thus reached. It has 

 nothing to do with the true process of gastrulation, but re- 

 presents a transient condition, which was perhaps acquired 

 in connection with the mechanism of hatching. 



Fig. 4. — Attached gastrula-stage of Sycandra raphanus (after F. E. Schulzf). 

 Ec, ectoderm; En, entoderm; in, gelatinous secretion between the two layers 

 (remnant of the cleavage cavity). 



When the embryo is hatched the invaginated part (ecto- 

 derm) returns to its former position, and an elongation in 

 the direction of the chief axis follows. The ovate swarm- 

 ing stage now reached is known as the amphiblastula 

 (Fig. 3 A). It consists of histologically differentiated 

 halves. The half of the body directed forwards in swimming 

 is composed of tall columnar flagellate cells, whereas the 

 large granular cells of the posterior half of the body bear no 

 fl.agella. Within is seen the considerably reduced cleavage 

 cavity (csy. 



' [According to recent investigations of Dexdy (Appendix to Literature 

 on Porifera, No. II.), small cells, which perhaps become mesoderm cells, 

 make their appearance in this cavity at an early period.] 



K. H. E. C 



