206 



FORMATION OF THE LAYERS. 



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 open- 

 ing 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 gastrula 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 invagiuation 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 segmeutatiou 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 

 believed by Hatschek to assist in absorbing the albuminous fluid in which 

 the eggs are suspended. These large cells are eventually covered by the 

 normal epiblast cells and subsequently disappear. In both these types 

 the hypoblast cells undergo, during their invagination, peculiar changes 

 connected with their nutritive function. 



In Euaxes (Kowalevsky) the seg- 

 mentation is far more unequal than 

 in the other types ; a typical epibolic 

 invagination takes place (fig. 140), and 

 the blastopore closes completely along 

 the ventral surface. 



In all the oligoehsetous types, 

 with the exception of Euaxes, where 

 the blastopore closes completely, 

 the blastopore becomes, or coin- 

 cides with the mouth. In Serpula 

 it is stated (Stossich), as we have 

 seen, to coincide with the anus : 

 a statement which receives con- 

 firmation from the similar state- 

 ments of Willemoes-Suhm (No. 

 358). It is necessary either to 



FIG. 140. TRANSVERSE SECTION THROUGH 

 THE OVUM OF EUAXES DURING AN EARLY 



STAGE OF DEVELOPMENT. (After Kowa- 



levsky.) 



cp. epiblast ; ms. rnesoblastic baud ; 

 liy. hypoblast. 



