456 THE EVOLUTION OF MAN. 



layer. These two medullary swellings coalesce over the 

 furrow, thus forming a tube ; in this case also, this nerve 

 tube or medullary tube is originally open in front, but 

 closed behind. Again, in the Ascidian larva also, the per- 

 manent mouth-opening is a new formation, and does not 

 originate from the primitive mouth of the Gastrula; the 

 latter closes, and in its neighbourhood the future anal 

 opening is formed by inversion from the outside, at the 

 opposite end from the opening of the medullary tube (Plate 

 X. Fig. 5, a). 



While these important changes are taking place, exactly 

 in the same way as in the Amphioxus, a tail-like appendage 

 grows out from the posterior end of the larval body, and 

 the larva curls itself within the spherical egg-covering in 

 such a way that its dorsal side projects, while the tail is 

 bent back upon the ventral side. In this tail now de- 

 velops a cylindrical cord, composed of cells, the anterior 

 end of which extends into the body of the larva between 

 the intestinal and the medullary tubes : this is the chorda 

 dorsalis, an organ which, except in this one case, is found 

 only in Vertebrates, and of which no other trace is to be 

 seen in Invertebrates. Here, again, the notochord consists, 

 at first, of a single row of large bright cells (Plate X. Fig. 

 5, cfi); afterwards it consists of several cell-rows. So, too, in 

 the Ascidian larva, the notochord develops from the middle 

 portion of a cell-stratum, the side portions of which become 

 tail-muscles, and which can, therefore, only be the skin- 

 fibrous layer. At the same time, a cell-stratum splits otf 

 from the intestinal wall, which afterwards forms the heart, 

 the blood and the vascular system, and also the intestinal 

 muscles. This is the intestinal-fibrous layer. 



