ANATOMY AND DEVELOPMENT. 197 



The Ascidian larva was known long before this time, 

 and the external features of its metamorphosis were de- 

 scribed in 1828 jointly by AUDOUIN and MILNE-EDWARDS, 

 to whom the discovery of the free-swimming larva is due. 

 Furthermore, the internal structure of the tailed larva, 

 and even the histological structure of the axial rod of the 

 tail, was described with some accuracy by KROHN in 1852, 

 but in ignorance of the details of the embryonic devel- 

 opment, he was unable to give the right morphological 

 interpretation to the various parts, and did not identify 

 the axial rod with the notochord of the higher forms. 



Segmentation and Gastrnlation. 



The segmentation of the egg, the formation of a hollow 

 one-cell-layered blastula, and the flattening and subse- 

 quent invagination of one side of the blastula to form 

 the two-cell-layered gastrula, take place on a plan so 

 essentially similar to what has been described above for 

 Amphioxus that it is not necessary to dwell at length 

 upon them here. Suffice it to point out that the segmen- 

 tation of the Ascidian egg takes place typically, according 

 to VAN BENEDEN and JULIN, on a strictly bilateral plan. 

 That is to say, when the ovum has divided into two 

 blastomeres, right and left, each blastomere represents 

 and will give rise to the corresponding half of the larval 

 body, and the descendants of the first two blastomeres 

 can be distinguished for a remarkably long time on each 

 side of the middle line of the embryo, a fact which is 

 highly characteristic of Ascidian development. 



After the gastrula has begun to elongate, and the blas- 

 topore has been narrowed down by the approximation of 

 its lips to a small aperture situated at the posterior dorsal 

 extremity of the embryo, the formation of the medullary 

 plate occurs. 



