THE P0LYCHJ3TA. 199 



canal, and the layer of large blastomeres, the hypoblast. Be- 

 tween the two, a mesoblastic layer appears, but the exact 

 manner of its origin is not known. On one face of the sac- 

 cular embryo the mesoblast becomes divided into a series of 

 quadrate masses, like the proto vertebrae of a vertebrate em- 

 bryo, disposed symmetrically on each side of a median line, 

 which corresponds with the future ventral median line of the 

 body. Along this line, the epiblast becomes thickened in- 

 ward, and the thickening is converted into the ganglionic 

 chain. At the same time, each quadrate mass of the meso- 

 blast is excavated by the development of a cavity in its in- 

 terior, whereby it becomes converted into a sort of sac. The 

 adjacent anterior and posterior walls of successive sacs unite, 

 and give rise to the mesenteric septa, while their cavities 

 become the chambers of the perivisceral cavity. The seg- 

 mental organs commence as cellular outgrowths from the 

 posterior face of each septum thus formed, and only subse- 

 quently become excavated and communicate with the exte- 

 rior. 



The development of the Earthworm, therefore, closely re- 

 sembles that of the Hirudinea, and more especially that of 

 the Medicinal Leech, in which the digestive cavity of the 

 embryo would seem to be formed, as in the Earthworm, by a 

 process which is, in a sense, invagination. It would appear 

 that the first-formed aperture is the mouth ; while the anus 

 is a secondary perforation; and the segmentation of the body 

 commences in the mesoblast. 



In the fresh-water Oligochceta, Euaxes and Tubifex, the 

 vitellus also becomes divided into large and small blastomeres. 

 The latter extend over the larger blastomeres, and form the 

 epiblast (= ectoderm). A mesoblast (= mesoderm), divided 

 into two broad longitudinal bands, is developed, and the oral 

 cavity is said to be formed by invagination of the epiblast 

 between the anterior ends of the two bands of the mesoblast. 

 In this case, the mouth in these genera is a secondary forma- 

 tion. The innermost layer of large blastomeres becomes the 

 hypoblast (= endoderm). 1 



The Polych^eta. — Except that the Polychceta are almost 

 invariably dioecious and marine, while the Oligochaeta are 

 monoecious, and inhabitants either of land or fresh water, it 



* Kowalewsky, " Embryologische Studien." (" Memoires de 1'Acad^mie de 

 St. P6tersbourg,'" 1361.) 



