I 



. GERM-HISTORY OF THE ASCIDIAN. 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, 

 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. 



Up to this point in the evolution of the Ascidian there 

 is no definite ground for assuming its near relationship to 

 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 

 Vertebrates, and which absolutely demonstrates the kinship 

 of the Ascidia and the Vertebrates. From the outer skin 

 (epiderTiiis) 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 Gastrula-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- 



