76 MONTROSE T. BURROWS 



plasma. In one preparation a piece of tissue, composed of three 

 myotornes and a segment of the dorsal cord of a 62 hour chick 

 embryo, was transplanted to a culture and allowed to grow for 

 four days before it was fixed and stained. The nerves growing 

 from the neural tube were collected into two groups, each of which 

 passed thiough different intersegmental spaces between the myo- 

 tomes. A dense mass of cells covered completely all the growing 

 nerves from the neural tube as far as the median border of the 

 muscle plates. The entire picture is similar to that of the segmen- 

 tal nerves as seen in an embryo chick. 



The growth of nerve cells aside from the development of nerve 

 fibres has been rarely observed. In one preparation of an isolated 

 piece of neural tube, a single layer of epithelial cells protruded a 

 short distance into the medium. These cells, however, showed no 

 evidence of division. No other evidence of increase in size or 

 thickness of the wall of the tube or outward extension of nerve 

 cells has been noticed. Occasionally in a network of nerve fibres, 

 cells showing characters slightly different from Schwann sheath 

 cells have been observed. Some of these cells, which have wandered 

 out into the plasma, stain deeply and have same general charac- 

 ters of the neuroblasts (fig. 11). However, at present sufficient 

 proof is not available for their exact identity. 



MESENCHYMATOUS CELLULAR OUTGROWTHS 



The mesodermal tissues studied in this work included muscle 

 of the heart and the myotomes and the interstitial connective 

 tissue cells. The growth of the muscle cells has been noted but 

 rarely and has consisted in the lateral extensions of short chains 

 of striated cells from the myotomes and the heart. The heart 

 muscle cells were further identified by having the same rhythmical 

 contractions as the portion of the heart from which they originated. 



The constant appearance and extensive growth in all prepa- 

 rations of the embryonic interstitial cells make them most 

 suitable for the study of cellular activity in these cultures. These 

 cells appear early, growing as a continuous layer over the surface 

 of the tissue or spreading out in various horizontal planes in the 



