520 THE ANATOMY OF INVERTEBRATED ANIMALS. 



IT 



Fro. H^.Phallusia mammiUata. Various stages in the development of the larvae. 

 (After KowalewBky.y 



I. The veskular morula, flattened and about to undergo irivagination :fh, blastocoele. 

 The large blastomeres constitute the hypoblast, the small ones the epiblast. 



n. The gastrnla with the blastopqre, or opening of invagination, eo: ch, the blasto- 

 meres which constitute the rudiment of the urochord ; dd, the remaining blasto- 

 meres of the hypoblast. 



III. A more advanced embryo: ch, dd, as before ; c. the epibiast; n, the nervous 

 layer of the neural cavity, which is now open only in front near ch. 



TV. An embryo with the caudal appendage distinct. The nerve-tube n is complete, 

 and the muscle-cells m are distinguishable. 



1 " Weitere Studien fiber die Entwickelung der einfachen Ascidien." 

 (" Archiv furMikr. Anat.," 1871.) 



