388 EMBRYOLOGY 



line of the collar. The cavities, however, which occur in 

 the central nervous system of the adult animal, are not to 

 be referred to a formation comparable with the neural tube 

 of the Vertebrata, but arise in the cell-layer which was split 

 off from the ectoderm, in all probability by the appearance 

 of fissures. However, at the end of the central cord, where 

 it merges into the indifferent cells of the ectoderm and 

 where the latter is considerably thinner, a kind of folding 

 process seems to take place; at this point also the lumen of 

 the central cord is said to communicate witli the outside 

 world (neiiropore ?) . There seems to be no relation between 

 the doi'sal groove and the blastopore ; for the groove does 

 not extend so far back. A direct connection with tlie 

 conditions occurring in the Chordonia is therefore not in- 

 dicated by this (comp, infra). ^ Like the chief, central parts 

 of the nervous system, its peripheral portions are also 

 differentiated from the lower cell-layers of the ectoderm, 

 which, according to Bateson, everywhere exhibits large 

 accumulations of sensory cells. 



General Considerations. — The external resemblance 

 of the Tornaria to the Echinoderm larvas and the oc- 

 currence of tbe water- vascular vesicle, opening out by 

 means of a dorsal pore, have caused Balanoglossus to be 

 brought into relation with the Echinodermata. Corre- 

 spondingly, the acorn, the lining of which is supplied by 

 the so-called water-vascular vesicle, has been explained as 

 the last remnant of the water-vascular system, as the single 

 remaining ambulacral tentacle. The nature of the skin, 

 provided with calcareous structures, is, in addition to the 

 water-vascular system, characteristic of the Echinodermata. 

 The entire absence of calcareous bodies in Balanoglossus 

 and the different condition of the skin, together with 

 the other peculiarities in the entire structure of the body, 



' [According to Spengel's description, it must be assumed that the 

 account given by the previous observers does not at all relate to the first 

 fundament of the nervous system, and that in its formation, which takes 

 place at an early period, there is no invagination. Differentiations of 

 the ectoderm without any invagination give rise to the nervous system, 

 which only subsequently sinks in deeper.— K.J 



