164 EMBRYOLOGY 



small pigment spots (Fig. 76 A). Under tliem the brain is 

 established somewhat later in the form of two club-shaped 

 bodies (Fig. 7G B). These bodies develop as ectodermal 

 thickenings, which afterwards sink deeper and by means of 

 a broad commissure unite into the common mass which they 

 constitute in the adult. The two longitudinal nerve-trunks 

 arise from them by means of a backward gi'owth. Two cell- 

 growths, which perhaps are to be explained as parts of the 

 water- vascular system, make their appearance as ectodermal 

 structures in the posterior portion of the ellipsoidal embryo 

 (Fig. 76 A and B, Ex). 



The development of the intestine takes place b}- the 

 abundant multiplication of the upper and lower entodermal 

 cells. In Fig. 76 A the embryo appears filled with the 

 mass of central entoderm cells which have been metamor- 

 phosed into food-yolk. The small entoderm cells are dis- 

 tributed over the surface of these ; they penetrate between 

 the yolk-spheres, the substance of which they dissolve, and 

 are finally converted into intestinal epithelium. This takes 

 place in the following manner : scattered -entoderm cells 

 surround a mass of yolk which has become reduced in size 

 by disintegration, and, as they begin to absoi^b this, form a 

 short tube, which unites with other intestinal cavities that 

 have arisen in the same way (Selexea). When, finally, the 

 intestine, with its branches, has arisen in this manner, the 

 embryo acquires the general appearance of the adult worm 

 (Fig. 76 B). The mouth arises at the place of the pre- 

 existing blastopore from an invagination of the ectoderm, 

 which fuses with the wall of the intestine. Fig. 7Q C shows 

 this condition in SfyJochns. The ectoderm supplies the 

 epithelial lining of the pharynx and pharyngeal pocket, the 

 musculature of which arises from the mesodermal elements 

 that are found massed in large numbers in the region of the 

 invagination (Fig. 76 C). 



According to Hallez, as well as Selenka, the mesoderm 

 continues its development from its earliest beginnings by 

 the outgrowth of tlie primitive mesoderm cells into four 

 mesodermal bands j)laced crosswise ; these fuse with one 

 another as soon as their cells become moi'e numerous, and 



