182 



BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



2. Splitting of an Elongated Protoplasmic Body (cord or tubule). We 

 may recognize here two sub-processes : (a) splittiug at the tip, and 

 {b) splitting along the whole length. 



a. As is well known, nerves grow out as blunt stalks which repeatedly 

 divide at their ends. Paterson has shown that some of the spinal nerves 

 split at the somato-splauchnic angle, as shown in Figure 11. His ('88, 



*''*!>*. 



Fig. 11. 



Arch. f. Anat., Jahrg. 1887, p. 376) shows in many cases that the split- 

 ting takes place where the end of the growing nerve strikes a rigid organ. 

 Thus, he says, when the third branch of the trigeminus strikes Meckel's 

 cartilage, it divides into the ramus lingualis and the ramus mandibnlaris; 

 and when the hypoglossal meets the jugular vein, it divides into its de- 

 scending and lingual branches. The way in which the rigid organs act 

 to produce splitting is thus explained by His. Pre-existing resistant ob- 

 jects may be considered as the " Motivs " of division ; for as a nerve 



Fig. 11. Cross section of a mouse embryo in the lumbar region, sliowing the 

 splitting of the spinal nerve at the upi)er angle of the coelom. After Paterson, '91, 

 Trans. Roy. Soc. London, CLXXXL, Fig. 5. 



