520 



THE ANATOMY OF IXVERTEBRATED ANIMALS. 



Fig. 149.— P/i all 'usia mammiMata.—Yar\o\\$ stages in the development of the larvae. 

 (After Kowalewsky.V 



I. The vesicular morula, flattened and ahout to undergo invagination : fh. blastcccele. 

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



II. The L r astrnla with Hie 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 cavity, which is now open only in fror.t near ch. 



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

 and the muscle-cells m are distinguishable. 



lU Weitere Studien uber die Entwiekelung der einfachen Ascidien." 

 ("ArchivfurMikr. Anat," 1871.) 



