520 



THE ANATOMY OF IXVERTEBRATED ANIMALS. 



CM 



2?m 



Fig. 149. — PhdOusia mammillata.—YaTiom stages in the development of the larvae. 

 (After Kowalewsky.) 1 



I. The vehicular morula, flattened and about to undergo invagination :fh. blastocoele. 

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



II. The gastrula 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 cavity, which is now open only in front near ch. 



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

 and the muscle-cells m are distinguishable. 



lU Weitere Rtudien fiber die Entwiekelung der einfachen Ascidien." 

 ("ArchivfurMikr. Anat.," 1871.) 



