STUDIES ON THE PEOTOCHOEDATA. 333 



at first a bilateral origin, but its left half becomes arrested at 

 an early stage of development, and is then modified into the 

 praeoral pit. The essential part of this praeoral body-cavity in 

 Amphioxus is of course the epithelium lining it. 



The first diflFerentiated hypoblastic structure that comes 

 behind the praeoral or anterior body-cavity in Amphioxus is 

 the endostyle, which, while being slightly oblique and asym- 

 metrical on the mright side (author, 33), has essentially a 

 dorso-ventral position. As to the general homology of the 

 endostyle in the two groups of the Cephalochorda and the 

 Urochorda no one would think of entertaining a doubt. In 

 both groups we have shown that its primary position is dorso- 

 ventral and at right angles, or nearly so, to its definitive posi- 

 tion, and it lies in both cases immediately posterior to the 

 prseoral lobe. There is, therefore, at once strong presumptive 

 evidence that the praeoral lobes or anterior body-cavities are 

 themselves homologous in the two groups, and this, I am con- 

 vinced, is the case. 



In the development of the mesoderm in the Ascidians it 

 does not come to a formation of coelomic pouches, although the 

 latter was affirmed by van Beneden and Julin for Clavelina, 

 but contradicted by Davidoff for the same form ; and I have 

 also been unable to find any archenteric pouches in the 

 embryos of Cynthia papillosa, which spawned in the tanks 

 at Naples in the month of November. Shortly after its forma- 

 tion at the expense of the hypoblast, the mesoderm in the 

 Ascidian embryo, except in the tail, breaks up into a system 

 of loose mesenchyme consisting of round cells scattered in the 

 general body-cavity. 



It would, therefore, be unreasonable to expect to find a 

 mesodermic epithelial lining to the anterior body-cavity or 

 cavity of the praeoral lobe, when there is neither splanchno- 

 pleur nor somatopleur in any other region of the body. We 

 should expect to find and do find that the praeoral lobe con- 

 tains scattered mesodermic cells, the bilateral origin of which 

 has been described above. The difl'erences in the origin and 

 character of the mesoderm of the anterior body-cavity in the 



