252 



EMBRYOLOGV 



as soon as the larva has located itself in the intestinal mus- 

 culature of the intermediate host. The developmental pro- 

 cesses proceed from the central part, the so-called embryonic 

 nucleus, for it is this which contains the formative material. 

 According to the observations of Leuckart, it is differenti- 

 ated into four groups of cells lying one behind the other 

 (Fig. 114 A). The hindermost of these four groups soon 

 acquires a greater volume, sending out a peripheral layer, 

 which spreads out in front and on the sides, and encloses 

 the other groups, with the exception of the most anterior 

 one (Fig. 114 A). The Echinorhynchus is formed for the 



most part out of these cell 

 groups. The most anterior is 

 said to become the proboscis, 

 the second the ganglion, the 

 third, which soon divides into 

 two, the sexual glands, and, 

 finally, the fourth the sexual 

 ducts. The cell-layer which 

 surrounds the groups subse- 

 quently splits into two layers, 

 which were treated by Leuc- 

 kart as answering to the soma- 

 tic and splanchnic layers of 

 the mesoderm. In the absence 

 of the intestine, the splanchnic 

 layer would be represented by 

 the so-called ligament and the 

 proboscis- sheath only, both of 

 which structures arise from it. 

 The somatic layer, after it has 

 separated farther from the 

 splanchnic, and has left the 

 body cavity between them, 

 would form the musculature 

 of the body, whereas the epidermis arises directly from the 

 larval skin. When the internal formative processes have pro- 

 gressed sufficiently to allow of it, the cuticula of the larva 

 is alone cast off. A new cuticula then arises. During these 



Fi6. lU.—A, B, two larvae of Echi 

 norhynchus jiroteus, in which the 

 " embryonic nucleus " is already un- 

 dergoing its metamorphosis (after 

 Lkuckabt). r, proboscis; rt, pro- 

 boscis-sheath ; 11, ganglion ; g, funda- 

 ment of the genital glands ; I, ducts 

 of the genital system; m, the cell- 

 layers which are destined for the 

 formation of the musculature. 



