DEVELOPMENT OF THE NEMATOIDA. 309 



tirict, the formation of the future head being always preceded by 

 that of the caudal extremity. With the extension in length, the 

 thick club-like creature becomes constantly more slender, the 

 globules smaller, the skin as if covered with granules, the interior 

 filled with similar globules, and, finally, a perfectly worm-like, 

 moving creature, the perfectly formed embryo, is produced from it, 

 which at last perforates the egg-shells (vitelline membrane and 

 chorion), and then swims freely about upon the field of vision. In 

 many species of worms, but not in all, the vitelline membrane is 

 extended by the growth of the embryo, and with it the enveloping 

 stratum of albumen and the chorion. How long a period elapses 

 between the fecundation and this moment cannot be stated, but 

 in many species the process is only concluded after a long time 

 (in Ascaris lumbricoides about twelve months). The structure of 

 the young worm, which resembles its parents, is usually very 

 simple. We only observe a granular mass (remains of vitelline 

 globules). The empty oesophagus, the interior of which is 

 indicated by two lateral lines, with the appended empty stomach, 

 the cavity of which is often triangular, and the first commence- 

 ment of the intestine behind the stomach, can be distinctly 

 recognised. The space from this to the caudal extremity is filled 

 with fine grains, and forms the foundation for the intestine. An 

 anus is not yet visible; even the mouth appears still to be 

 deficient at this period. In the round-worms furnished with 

 weapons on the head, these organs are of course formed simul- 

 taneously, and are ready before the embryos quit the egg-shells 

 arid start on their migrations. 



We might still mention here the question of the embryologists, 

 whether the globules of segmentation just referred to, with the 

 enclosed nucleated embryonal-cells, are to be regarded as com- 

 pound cells, analogous to the ganglionic globules, or to the un- 

 fecundated egg, and to be explained in the manner of cell- 

 formation ? Kolliker and others have shown that this is not the 

 case, because the cell-formation always precedes the former as the 

 causative momentum, although the enveloping of the embryonal 

 cell with the yelk presents a very important momentum of 

 development. According to Kolliker, therefore, we are compelled 

 to suppose that the remarkable phenomenon which we call 

 segmentation, is the expression of an attraction which is exerted 

 by the embryonal cells upon the surrounding yelk-mass. 



The metamorphoses just described take place partly even in 



