381 



A nuclear membrane (n. m) is easily defined and possesses the 

 staining properties of chromatin. It appears granular and more or 

 less interrupted. In places the cytoplasmic reticulum is intimately 

 related, yet I have been unable to trace the threads of this reticulum 

 directly into either the nuclear membrane or nuclear network. 



Each nucleus possesses, at this time, one to four true nucleoli or 

 plasmosomes (p) which are characterized by their spherical form, shght 

 affinity for chromatic stains, peculiar refraction and sheath of chromatic 

 granules. These plasmosomes are fairly uniform in size, but occupy 

 no constant position with reference either to the axis or periphery 

 of the nucleus, being irregulary scattered, much as shown in Fig 1. 



The chromatic nucleoli, net knots, or more properly karyosomes 

 (Jc) are variously distributed in the different nuclei. In many there 

 is a peripheral zone which is comparatively free from these chromatic 

 masses. In most nuclei there is no characteristic arrangement either 

 with reference to the axial or peripheral portion of the nucleus. The 

 karyosomes are usually irregular with numerous processes which extend 

 along the linin threads (l) and form anastomoses with adjoining karyo- 

 somes as shown in the figure. 



In the 17 mm stage the fibrillae fill the notochordal half of the 

 muscle cell while in the 26 mm stage there are no longer observed 

 unfibrillated areas in those cells which make up the body of the 

 myotome. At the upper outer and lower margins of the myotome, 

 many muscle cells are found which are not as yet entirely fibril- 

 lated, indicating that in these locahties new mnscle cells are being 

 added. 



The nuclei in the 15 — 17 mm embryos lie in close contact with 

 the fibrillated areas. With the continued differentiation of fibrillae they 

 pass farther and farther toward the outer portion of the muscle 

 cell and finally in the 26 mm stage they lie at the periphery just 

 beneath the sarsolemma. In most cases the nuclei are flattened on 

 the side next the fibrillae so that a large extent of the nuclear sur- 

 face is brought closely in contact with the portion of the cell in 

 which cytoplasmic activity is greatest. The nuclei at this time have 

 a somewhat different outline from those observed in the earlier stages 

 in that they are more elongated with more acute ends, as shown in 

 Fig. 2. 



The nuclear membrane can no longer be distinguished since it is 

 lined by a layer of chromatin made up of karyosomes which are so 

 closely applied that no line of demarcation is present. The linin 

 network {1} shows a decided change in character. Its meshes are 



