ANATOMY AND DEVELOPMENT. 21 J 



above difference in the vacuolisation of the notochordal 

 tissue in Amphioxus and the Ascidian larva would resolve 

 itself into saying that the secretion was retained inside 

 the cells in the one case, and deposited outside them in 

 the other. 



Mesenchyme and Body-cavity. 



The endoderm cells of the tail, which formed at first a 

 solid cord below the notochord, have now become con- 

 verted into loose corpuscles, which have mostly floated 

 out of the tail into the hinder portion of the body-cavity, 

 and have become indistinguishable from the mesoderm- 

 cells. The latter are beginning to lose their compact dis- 

 position in the form of the two mesodermic bands, espe- 

 cially in the hinder region, and to be scattered about in the 

 body-cavity. 



The body-cavity of the young Ascidian is not unre- 

 servedly homologous with that of Amphioxus, on account 

 of this remarkable behaviour of the mesoderm. The 

 cavity does not arise in the midst of the mesoderm by a 

 splitting apart of its component cells, but it is simply 

 produced by a separation of the endoderm from the ecto- 

 derm, the two layers being at first in contact at the sides 

 and below ; in fact, everywhere, except where the dorsal 

 nerve-tube intervenes. 



In the cavity thus produced between ectoderm and 

 endoderm the mesodermic bands at first lie freely, and 

 then their component cells break away from their compact 

 association and float about the cavity in the form of 

 scattered corpuscles, known collectively as mesenchyme. 



This mesenchyme later gives origin to the muscula- 

 ture of the body proper of the Ascidian, and also to 

 the definitive blood-corpuscles, genital organs, and renal 



