102 GEORGO ORIHAY SHINJI 



A. The ventral nerve cord. The rudiments of the ventral 

 nerve cord arise from the ectoderm layer of the ventral plate, 

 the origin of which has already been described. At first the 

 ectoderm cells forming the ventral wall of the amniotic cavity 

 are all similar in appearance, size, staining reaction, etc. They 

 are all cylindrical, almost spindle-like in shape (fig. 103). 



Later, however, on each side of the long axis of the embryo, 

 a few large spherical cells are formed within the ectoderm layer 

 by mitotic division. Since, as will be shown later, all of the 

 nerve cells arise from these large spherical cells, the name, 

 neuroblasts, is applied to them. The remaining cells in the 

 ventral ectoderm do not contribute to the nervous system, but 

 go to form the body wall, and consequently they are called the 

 dermatoblasts. In like manner, the term neurogenic area and 

 dermal layer are, respectively, used to designate the portion of 

 the area containing the neuroblasts and that without them. 



The number of the neuroblasts increases by a further mitotic 

 division of ectodermal cells. In figure 104 four such neuro- 

 blasts are shown, two on either side of the middle line, and in 

 addition to these, a division figure of an ectoderm cell is given. 



In figure 107, which represents a transverse section through 

 the first thoracic appendages, the separation of the nerve rudi- 

 ments from the dermal layer is pronounced. The ectoderm layer 

 now becomes so thin that it is everywhere but one cell in thick- 

 ness. On the other hand, the nerve rudiments are several cells 

 thick. A layer of cells next to the dermal ectoderm in each of 

 the body segments consists of six large cells. Each of these 

 neuroblasts is usually seen in a different phase of mitosis. Dorsad 

 to the neuroblasts, there are about five horizontal rows of smaller 

 cells, which stain much darker than the surrounding neuroblasts, 

 the dermatoblasts, or the mesoderm cells. Nevertheless, these 

 darkly staining cells are the true daughter cells of the neuro- 

 blasts and are called ganglioblasts, for they form the ganglia. 

 All ganglioblasts are of the same size and are in the resting 

 stage. These facts indicate that they do not divide after they 

 become once separated from the neuroblasts. 



