THE PRINCIPLES OE EMBRYOLOGY tf \ 



a neural syncytium, which we may call the host, which carries with 

 it in its meshes a number of free cells not connected with the nervous 

 system. If, then, we confine our attention to the host and trace back 

 this neural syncytium to its beginnings in the embryo, we see that, 

 from the very nature of the neuro-epithelial couple, each epithelial 

 moiety must approach nearer and nearer to its neural moiety, until 

 at last it merges with it ; the original neuro-epithelial cell results, 

 and we must obtain, as far as the host is concerned, a single-layered 

 blast ula as the origin of all Metazoa. It follows, further, that there 

 must always be continuity of growth in the formation of the host, 

 i.e. in the formation of the neuro-epithelial syncytium ; that there- 

 fore cells which have been previously free cannot settle down and 

 take part in its formation, as, for instance, in the case of the formation 

 of any part of the gut- epithelium or of muscle-cells from free-living 

 cells. 



Further, since the neural moiety is the one element common to 

 all the different factors which constitute the host, it follows that the 

 convergence of each epithelial moiety to the neural moiety, as we 

 pass from the adult to the embryo, is a convergence of all outlying 

 parts to the neural moiety, i.e. to the central nervous system, if there 

 is a concentrated nervous system. Conversely, in the commencing 

 embryo the place from which the spreading out of cells takes place, 

 ■i.e. from which growth proceeds, must be the position of the central 

 nervous system, if the nervous system is concentrated. If the nervous 

 system is diffuse, and forms a general sub-epithelial layer, then the 

 growth of the embryo would take place over the whole surface of 

 the blastula. 



Turning now to the consideration of the second group of tissues, 

 those that are not connected with the central nervous system, we 

 find that they include among them such special cells as the germinal 

 cells, free cells of markedly phagocytic nature, and cells which were 

 originally free and phagocytic, but have settled down to form a 

 supporting framework of connective tissue, and are known as plasma- 

 cells. In the embryo we find also in many cases free cells in the 

 yolk, forming more or less of a layer, which function as phagocytes 

 and prepare the pabulum for the fixed cells of the growing embryo ; 

 these cells are known by the name of vitellophags, and in meroblastic 

 vertebrate eggs form somewhat of a layer known by the name of 

 periblast. Such cells must be included in the second group, and, 



