296 Robert Thompson Young, 



d) Musculature. 

 Excepting the parenchyma, the earliest of the tissues to be 

 cliiferentiated is the musculature. At so early a stage of develop- 

 ment as that represented in Fig. 1, there is seen in the wall of 

 the larva two indefinite layers of fibres, one running longitudinally 

 and the other transversely. They are imbedded in a network of 

 fibres which run in all directions and are only to be distinguished 

 from the latter by the definitive direction in which they run. These 

 layers are the rudiments of the future sub-cuticular muscles. In the 

 close network of parenchyma fibres making up the wall of the larva, 

 and of which network they are as yet a part, it is impossible to trace 

 a connection between these embryonic muscle fibres and the cells 

 with which they are connected. Nor is it possible at this stage to 

 differentiate these cells — the so-called "myoblasts" ^) — from any 

 other of the parenchyma cells. In fact, as I hope to show later, 

 the muscles in the genetic condition pertain no more to one cell or 

 one group of cells than to another. They merely form a part 

 of the general parenchyma syncytium. In a somewhat 

 later stage than is illustrated in Fig. 1 the bladder wall is more 

 distended and the cells consequently more widely separated. Here 

 it is possible to pick out single neuro-muscular cells and trace their 

 connection, on the one hand with the developing muscle, and on the 

 other with the parenchyma. Such a cell is illustrated in Fig. 43, 

 where one of its peripheral processes may be seen in direct conti- 

 nuity with a muscle, ni. Centrally the cell body is very plainly in 

 direct continuity with the parenchyma. The cytoplasm in this cell 

 is finely fibrillar, the fibrillae running in various directions, but 

 being oriented in a general way parallel to the long axis of the 

 cell. These fibrillae extend into the cell processes where they are 

 gathered together more densely and form, so far as I have been 

 able to resolve them with a magnification of 2850 diameters, the 

 ultimate units of the muscles. The nucleus presents a fibrillar 

 structure more dense than that of the cytoplasm. It contains one, 

 deeply stained, homogeneous, round mass of chromatin -) and a few 



1) The term "myoblast" is an unfortunate one implying as it does 

 that these cells are the progenitors of the muscle fibres, which is only 

 partially true. It is preferable to call them neuro-muscular cells, for 

 reasons which will be explained hereafter. 



2) For reasons to appear later, this structure will not be considered, 

 as has been commonly done hitherto, a "nucleolus". 



