MORPHOLOGY OF EYE MUSCLE NERVES 39 



it penetrates with its neurofibrils. At this stage the motor nerve 

 anlage in Anamnia is non-cellular while in Amniota it is cellular. 

 The cause of this difference is as follows : In case the nerve -an- 

 lage makes its appearance while the germ layers and the an- 

 lagen of the organs are still epithelial as in the Anamnia, the 

 peripheral neurofibrillar tract lies in nuclear-free and net-like 

 plasmodesms, which connect the related epithehal surfaces with 

 each other. If, on the other hand, a connective tissue is already 

 developed, then such nerve fibers extend through a variable 

 number of cell-bodies. In the first case the nerve appears 

 primarily without nuclei, while in the second it appears as a nu- 

 cleated structure. The difference is due to a difference in the 

 relative time of differentiation of the neuroblasts. 



3. The third stage is a transition to the condition in the adult 

 nerve with its cellular neurilermna sheath, and arises through 

 the emigration of medullary cells into the fibrillar nerve and 

 their differentiation as sheath cells. 



Held summarizes his views regarding the histogenesis of motor 

 nerves as follows: 



The origin of motor nerves rests upon the peripherally directed 

 growth of the specific cell substance of definite neuroblasts of the neu- 

 ral tube and which proceeds in the direction of the chief cell axis of 

 the neuroblasts, and, for definite but unkown reasons, transcends 

 the outer boundary of the embryonic neural tube. A stage of devel- 

 opment precedes the formation and growth of the nervous substance 

 itself, by means of which the neural tube and its motor neuroblasts, 

 through net-like arranged paths of a simpler and not yet neurofibrillar 

 substance, is brought into coimection with the peripheral muscle 

 anlage by complicated processes of outgrowth. 



Such connection paths, which in Anamnia are simple and epithelial 

 and in the Amniota, on the contrary, are a complicated connective 

 tissue, are in the Hensen's sense of the word 'used' by, and in some 

 sort of an unknown manner fused with, the nervous substance growing 

 from the neuroblasts. The growth of the nerves to the terminal organ 

 does not proceed in the liquid-filled vacuoles of the intercellular spaces 

 as affirmed by His, since the growing points of the nervous substance 

 are connected both laterally and at the outer extremity with the farther- 

 reaching plasmodesms of the surrounding tissues. Later, by means 

 of complicated processes of multiplication and movement of medul- 

 lary cells, which wander secondarily into the primarily non-nuclear, 

 or nucleated, nerve-path as cell-elements organically connected with 

 it, the motor nerve becomes the multinucleated strand supplied with 



