So-called Mesectoderm in Petromyzon. 



467 



an epithelial character, but make up a simple network of cells with the 

 nervous cells coming forth from the nerve ridge, by which the epidermic 

 ectoderm is connected with the walls of the medullary canal. This network 

 corresponds to that part of the mesectoderm which Koltzoff distinguishes as 

 the dorsal division from the epithelial ventral part of it. The dorsal division 

 is accordingly confined to the preotic region, while the ventral division is to 

 be traced uninterruptedly to the corresponding part in the postotic branchial 

 region and constitutes one continuous structure from the mandibular arch to 

 the hindmost visceral arch. 



The network forming the dorsal division becomes gradually more 

 complicated, owing to further growth of both the nervous and mesectodermic 

 cell-strings ; so that the elements of both' kinds are not easily distinguished 

 from each other, as Koltzoff complains. The cells of the dorsal division ought 

 not, therefore, to be overlooked at the earlier stage of their appearance, a* 

 phase in which the nervous cells coming downwards from the medullary roof 

 and the mesectoderm cells arising from the lateral layer of the somites do not 

 as yet meet with each other. Then, in the following stages, both kinds of cells 

 are not very difficult to trace into the points from which they start respec- 

 tively. At the dorsal corner of the lateral layer of the somites active cell- 

 divisions can be observed which are repeated during the course of develop- 

 ment for not less than 12 hours, and the course taken by the resulting cells 

 in passing into the network is not difficult to make out. The nerve-cell 

 strings passing downwards almost vertically can also be traced with 

 certainty. The nerve fibres developed from these strings associate with 

 those from the ganglion of epidermic origin and make their way between 

 the mesectoderm and somitic layer of the lateral plates, as was obvious 

 already in an embryo of the eighth day. 



The foremost somite gives off the mesectoderm cells not only from its 

 outer wall, as in the following somites, but also from the anterior wall of the 

 blind pocket by which the first somite on each side stands in connection with 

 its fellow. The cells from both sources fill up the space between the 

 ectoderm and the somite and the space below the anterior extremity of the 

 brain. The free cells lying in close contact with the ectoderm are trans- 

 formed into the epithelial mesectoderm, which can by no means be distin- 

 guished from the ventral division in the posterior region and is continuous to 

 it ; those inside are developed into the network of muscle fibres which 

 occupies the interior of the upper lip in later stages. 



While the outer walls of the first, second, and third somites and also the 

 anterior wall of the first are broken up into the mesectoderm cells, the inner 

 walls of these three somites, which correspond to the muscle plate of the 



