178 bulletin: museum of comparative zoology. 



occupy the position of those from which, at a slightly more advanced 

 stage, the neuraxons of the oculomotor nerve take their origin, and 

 since there is, even at this stage, evidence of the outgrowth of their pro- 

 cesses from the neural tube, it follows that each group of cells is to be 

 looked upon as the developing nidulus of the oculomotor nerve of that 

 side. The niduli, then, at the very beginning of their development, are 

 in a decidedly ventral position. 



The neuroblasts on the right side of the median plane are more closely 

 grouped together than those on the left, and even under low powers of 

 the microscope stand out clearly from the rest of the medullary wall 

 because of their darkly staining cytoplasm. Each group lies close to the 

 external limiting membrane, projecting into the region of the marginal 

 veil. On the right, the external limiting membrane remains intact, no 

 processes of the neuroblasts having forced their way through it. On 

 the left (Plate 3, Fig. 8), however, the external limiting membrane has 

 been ruptured, and a part of the substance of the marginal veil protrudes. 

 In the parasagittal section of the ventral mid-brain wall shown in Fig- 

 ure 8 there are included only a few of the several neuroblasts making 

 up the nidulus. The letters n'bV designate a neuroblast with its cyto- 

 plasm drawn out, aud directed toward the break in the medullary wall. 

 Near it lie two cells, with cytoplasm tending in the same direction. 

 The neuroblast marked n'bl." shows a well-defined cytoplasmic process, 

 narrowing toward its peripheral extremity, which has evidently pushed 

 its way through the external limiting membrane. Of interest is a nu- 

 cleus (cl. med. mig.), plainly medullary, lying in the material which is 

 escaping from the neural wall through the aperture in the external limit- 

 ing membrane to which reference has been made. This medullary nu- 

 cleus appears to be making its way out of the neural tube at a very 

 early stage in the development of the oculomotor nerve. 



A little posterior to the oculomotor nidulus, at this stage, a few of the 

 first fibres of the ventral fibre-tract can be seen running caudad toward 

 the hind-brain. 



2. Eye Muscles. As early as this stage, the fundament of the poste- 

 rior rectus muscle has made its appearance, but since the series repre- 

 senting Stage II is more favorable for its study, description is deferred 

 until then. Its character and relations in Stage I have not been altered 

 in Stage II. 



Stage II. 



This stage is found in a series of seventy hours' incubation. 



1. Oculomotor Nerve. Cross-sections of the mid-brain show the oculo- 



