520 THE ANATOMY OF INVERTEBRATED ANIMALS. 



Fig. HQ.—Phallusia mammillata.—Yarions stages in the development of the larvse. 

 (After Kowalewsky.) 1 



I. The vesicular morula, flattened and about to undergo invagination :fh, blastocoelc. 

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



II. The gastrnla with the blastopore, or opening of invagination, eo: eh, the blasto- 

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

 meres of the hypoblast. 



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

 layer of the neural cavitv, which is now open only in front near eh. 



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

 and the muscle-cells m are distinguishable. 



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

 (" Archiv fur Mikr. Anat.," 1871.) 



