250 



EMBRYOLOGY 



Leuckart, takes place in Echinorhynchus proteus and angus- 

 tatus in such a manner that the spindle-shaped e^g is divided 

 at right angles to its long axis into a number of cells which 

 are not quite equal in size (Fig. 112 A, B). After five 

 blastomeres have been formed in this way, they are divided 

 in the direction of the long axis, and a rather irregular ar- 

 rangement of the cleavage spheres ensues (Fig. 112 C, 

 D). As the result of cleavage an epibolic gastrula is pro- 

 duced (Kaiser), the outer layer of which is formed of a 

 large number of polyhedral cells, whereas the inner layer 

 consists of much larger cells and encloses a remnant of yolk 

 in the centre. Even at this stage the embryo acquires its 

 armature. In the middle of every group of four contiguous 

 ectoderm cells is formed, as the product of their secretion, a 

 recurved booklet which protrudes 

 into the space bounded by the em- 

 bryo and the innermost protective 

 envelope. The entire surface of an 

 embiyo of Echinorhynchus gig as is 

 beset with small spines, and in ad- 

 dition five larger hooks are found at 

 the anterior end (Fig. 113 A). The 

 anterior end of the body, on which 

 they are located, can be retracted, 

 forming a funnel-shaped depression. 

 In Echinorhynchus angustatus it is 

 truncated, and five to six hooks are 

 always found on the disc thus 

 formed (Fig. 113 B). As in Echino- 

 rhynchus gigas, it also can be drawn 

 in. 



After the central yolk is entirely consumed, there begins 

 a process called by Kaiser histolysis. This consists in the 

 following changes: the boundaries between the cells dis- 

 appear, the bodies of the cells flow together, and the cell 

 nuclei move to the middle of the embryonic body, where 



Echinorhynchus gigas, which, however, have as yet been published only 

 in a preliminary way and without illustrations, but which nevertheless 

 afford an insight into the development of these forms. 



Fig. 113.—^, embryo of 

 Echinorhynchus gigas in the 

 embryonal membranes (eifi) ; 

 B, larva of Echinorhynchus 

 angustatus with the disc (s) 

 at the anterior end bearing 

 the armature (after Leuc- 

 kart) ; efc, •' embryonic nu- 

 cleus." 



