caepentee: development of the oculomotoe neeve. 185 



The fundaments of the dorsal rectus and dorsal oblique muscles, and 

 the common fundament of the anterior and ventral rectus muscles, are 

 now to be seen. These consist of small, local differentiations of meso- 

 dermal cells, which tend to become more closely associated, and, through 

 their activities, produce an abundance of cytoplasmic material ; this 

 material represents the first stage in the formation of the contractile 

 substance of the muscle cells. These muscle fundaments will be 

 described and figured in Stage V, where my sections are in planes more 

 favorable for showing their relations. Their positions in the later stage 

 are practically the same as in the present one. 



The fundament of the ventral oblique muscle cannot be distinguished 

 at this stage. 



It might here be stated that it has been impossible, in the chick, to 

 assign the eye-muscle fundaments to their respective somites by the aid 

 of the head cavities. These have not been present in the stages studied 

 owing to the fact that, in the chick, they appear to be obliterated very 

 early in development — much earlier than is the case in the embryos 

 of ducks (van Wijhe, '86 ; Eex, :00) terns, gulls and lapwings (van 

 Wijhe, '86). 



Stage IV. 



This stage occurs at about the one hundredth hour of incubation. It 

 is described from two series, one of one hundred hours, the other of one 

 hundred and one hours. 



1. Oculomotor Nerve. The oculomotor nidulus retains its extreme 

 ventro-median position. In fact it appears to have moved toward 

 rather than away from the median plane. Although the neural tube 

 has increased in size, the centre of the nidulus now lies nearly 50 micra 

 nearer the median plane than in the stage last described, being distant 

 only 105 micra from it. The ventral fibre tract has increased in thick- 

 ness, and, while the nidulus of the oculomotor extends into it, the lower 

 border of the nidulus does not lie as near the external limiting membrane 

 as in Stage III. Neuraxons from the nidulus, passing through the 

 ventral fibre tract — a I'egion free from nuclei — are frequently seen to 

 be accompanied by nuclei which have the rounded form of the indiffer- 

 ent elements in the nidulus dorsal to them, and of the "accompanying" 

 cells of the root of the oculomotor ventral to them. Farther out on the 

 trunk of the nerve, the grtat majority of the cells have become elongated) 

 though occasional round ones are to be observed. 



The oculomotor nerve pursues, as in the stage last described, a 



