520 



THE ANATOMY OF INVERTEBRATED ANIMALS. 



C/i& 



Fig. 149.— Phallusia mammillata.— "Various stages in the development of the larvae. 

 (After Kowalewsky.) 1 



I. The vesicular morula, flattened and about to undergo invagination : /A, blastoccele. 

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



II. The gastrnia with the blastopore, 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 epiblast; n, the nervous 

 layer of the neural cavil v. 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. 



l "Weircre Studien fiber die Entwickelung der einfachen Ascidien." 

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



