CH^TOPODA. 321 



which the segmentation is quite regular 1 . In other forms it is 

 more or less unequal. The formation of the germinal layers 

 has been far more fully studied in the Oligoch?eta than in the 

 Polychaeta, and though unfortunately the development is much 

 abbreviated in the former group, they nevertheless have to serve 

 as our type ; and unless the contrary is indicated the statements 

 in the remainder of the section apply to the Oligochaeta. The 

 segmentation is nearly regular in Lumbricus agricola (Kowa- 

 levsky) and results in the formation of a flattened blastosphere, 

 one of the sides of which is hypoblastic and the other epiblastic, 

 the hypoblast cells being easily distinguished from the epiblast 

 cells by their clearer aspect. An invagination takes place, 

 in the course of which the hypoblast becomes enclosed by the 

 epiblast, and a somewhat cylindrical two-layered gastrula is 

 formed. The opening of this gastrula at first extends over 

 the whole of what becomes the ventral surface of the future 

 worm, but gradually narrows to a small pore the permanent 

 mouth near the front end. The central cavity of the gas- 

 trula is lined by hypoblast cells, but the oral opening, which 

 leads by a narrow passage into the gastric cavity, is lined by 

 epiblast cells. 



The segmentation of Lumbricus trapezoides (Kleinenberg, No. 341), and 

 of Criodrilus (Hatschek, No. 339), is more unequal and more irregular than 

 that of Lumbricus agricola, and there is an invagination which is inter- 

 mediate between the embolic and epibolic types. 



The segmentation of Lumbricus trapezoides is especially remarkable. It 

 is strangely irregular and at one period the segmentation cavity communi- 

 cates by a pore with the exterior. Before the completion of the gastrula 

 stage the ovum becomes partially divided into two halves, each of which 

 gives rise to a complete embryo. The two embryos are at first united 

 by an epiblast cord which connects their necks (fig. 141 A), but this cord is 

 very early ruptured, and the two embryos then become quite independent. 

 Some of the peculiarities of the segmentation may no doubt be explained 

 by this remarkable embryonic fission. 



The gastrula opening in both Lumbricus trapezoides and Criodrilus is 

 placed on the ventral surface, and eventually narrows to form the mouth 

 or possibly (Criodrilus) closes at the position of the mouth. In Lumbricus 

 trapezoides the oral opening is at first lined by hypoblast, and in Criodrilus 

 is bounded anteriorly by three large peculiar epiblast cells, which are 



1 According to Willemoes-Suhm, Terebellides stroemii is also characterised by a 

 regular segmentation, 



B, II. 21 



