DISTOMDM OIRRIGERUM. 295 



in the new genus Astacotrenia^ not very far removed from 

 Astia. 



The development of the sexual form presents some features 

 of theoretical interest in connection with the doctrines of 

 heredity and of the germ-layers. The irregular and appa- 

 rently erratic nature of the development is very surprising. 

 An embryo consisting of a few loose blastomeres and yolk- 

 cells escapes from the egg-shell. Nothing of the nature of a 

 " hyaline membrane " or the stripping oif of an outer layer of 

 flat cells could be detected. The whole embryo may or may 

 not fall to pieces^ so to speak, and the separated blastomeres 

 are able to develop into the mature animal. More usually 

 the embryo grows to a considerable size, and then it may or 

 may not send out buds consisting of one or several blasto- 

 meres with perhaps a few yolk-cells attached. These buds 

 do not necessarily develop ; they may degenerate and form a 

 more or less continuous sheath around the central developing 

 embryo. The blastomeres remain of a considerable size 

 during the earlier stages of division and budding. Next the 

 blastomeres divide into quite small cells, and the embryo 

 becomes oval in outline, and ultimately somewhat bean- 

 shaped, and a thin irregular cuticle is produced. Even at 

 this advanced stage the embryo may apparently divide by 

 fission into two. The whole embryo consists of an apparently 

 perfectly homogeneous mass of small cells. There is next 

 secreted a very thick cyst- Avail, which is sometimes, although 

 not always, produced at some little distance within the body 

 of the embiyo. On account of the erratic nature of this 

 outer layer, which is by no means always formed, it is very 

 doubtful if it should be regarded as ectoderm. Clearly the 

 cells which become enclosed by the new cyst will alone form 

 the embryo, and therefore up to this period the general cells 

 of the embryo are not predestined to form any particular part 

 of the future body. The fundament of the vesicula seminalis 

 and surrounding cirrus-sac may, however, sometimes appear 

 even before the thick cyst is formed. 



At first the cells of the embryo in contact with the thick 



