GERM-HISTORY OF THE ASCTDIAN. 455 



on " Zellbildung." ^^^ Here, as in the Amphioxus, the germ- 

 vesicle (nucleus) of the egg-cell disappears in great measure 

 even before fertilization, while, after the latter process is 

 accomplished, the monerula, in consequence of the re-forma- 

 tion of a kernel, becomes a cytula. This breaks up by 

 primordial cleavage into 2, 4, 8, 16, 32 cells, and so on. By 

 continued total cleavage the morula forms the mulberry-like ^ 



heap of like cells. Within this a liquid accumulates, and 

 thus a globular germ-membrane vesicle is once more formed, i 



the wall of which consists of a single cell-stratum, the  



blastoderm (Plate X. Fig. 3), just as in the case of the < 



Amphioxus a true Gastrula, a simple Bell-gastrula (Plate X.  



Fig. 4), is formed from this blastula by inversion. j 



Up to this point in the evolution of the Ascidian there , 



is no definite ground for assuming its near relationship to j 



the Vertebrates ; for a similar Gastrula arises in the same \ 



way in the most diverse animals of other tribes also. Now, ^ 



however, comes an evolutionary process which is peculiar to I 



Vertebrates, and which absolutely demonstrates the kinship ' J 

 of the Ascidia and the Vertebrates. From the outer skin j 



{ejpidermiiis) of the Gastrula originates a medullary tube, 

 and, between this and the primitive intestine, a notochord 

 — organs which otherwise occur only in Vertebrates, and 

 are peculiar to them. The formation of this highly im- 

 portant organ takes place in the Gastrula of the Ascidian 

 exactly as in that of the Amphioxus. In the Ascidian also, 

 the oblong-round or oval Gastiula-body, which has but a 

 single axis, becomes flat on one side, on the future dorsal 

 side. Along the central line of this flat side, a furrow or 

 trench forms, the medullary furrow, and on either side of 

 this two parallel ridges or swellings arise from the skin- 



