ANNELIDA 319 



further cares for the brood by carrying about with it, at- 

 tached to its ventral side, the young after they have hatched 

 from the cocoon. The cocoon ordinarily contains a large 

 number of eggs, as many as twenty in the medicinal leech. 

 The Gnathobdellidas and Rhynchobdellidoe are distinguish- 

 able by the fact that the cocoon of the former is filled with 

 albumen, in which the eggs are found embedded, whereas in 

 the Rhynchobdellidae the cocoons lack the albumen, and the 

 much larger eggs lie in rows and in layers alongside and 

 above one another in great numbers, in Clepsine, for example, 

 as many as 200. Correspondingly the eggs of the Gnathob- 

 dellidae are small, and contain little yolk ; the embryos leave 

 the eggs at an early stage of development, and, like the 

 Oligochgeta, float as larval forms in the albumen of the 

 cocoon, by means of which they are nourished. Only after 

 several weeks do they quit the cocoon. The Rhynchob- 

 dellidfe, on the contrary, whose large, richly yolk-laden eggs 

 furnish to the embryos sufficient nourishment, do not break 

 through the egg-membrane until a much more advanced stage 

 of development and soon after also abandon the cocoon. 



1. Cleavage, Formation of the Germ-layers, and 

 Development of the Outward Form of the Body. 



A. Rhtnchobdellid^. 



The process of cleavage can best be followed in the Rhyn- 

 chohdellidas, on account of the larger size of the eggs, and 

 has been repeatedly studied in Clepsine. According to 

 "Whitman, three small blastomeres and a single larger one 

 are first pi'oduced by the formation of two vertical cleavage 

 planes, whose position indicates the subsequent orien- 

 tation of the body of the worm. The three smaller ones 

 mark the anterior end, the larger one the posterior end, of 

 the worm. Then four small blastomeres bud out at the 

 animal pole from the four large ones, whereby the familiar 

 stage of four macromeres and four micromeres is reached (Fig. 

 151 A). The further metamorphosis consi-sts in the separa- 

 tion of the posterior large blastomere into two of nearly the 



