188 bulletin: museum of comparative zoology. 



branches are given off immediately proximal to the large transitory- 

 ganglion. These branches can be followed for short distances into the 

 mesenchyme. 



As was stated in the description of Stage III, ganglion cells can be 

 found scattered along the nerve from the mesocephalic to the transi- 

 tory ganglion. These cells are more numerous near the mesocephalic 

 ganglion; in fact, the transition from ganglion to nerve is a very 

 gradual one, since so many ganglion cells have migrated outside the 

 true limits of the ganglion. Comparison shows that the ganglion cells 

 to be found in the mesocephalic ganglion are precisely like those of the 

 transitory ganglion. Both have relatively large, rounded nuclei con- 

 taining chromatin which is concentrated, for the most part, into one or 

 two large masses. Surrounding the nucleus, but lying mostly on one 

 side of it, is a considerable amount of finely granular cytoplasm, which 

 becomes drawn out to a blunt extremity, and stains a deep blue with 

 iron haematoxylin. The nucleus itself is much less deeply stained, 

 with the exception of the included chromatin particles, which take on 

 ,a dense, black appearance. Such ganglion cells are shown in Plate 2, 

 Figure 5, where o and P are cells from the transitory ganglion and y, a 

 ■cell from the mesocephalic ganglion. These ganglion cells are plainly 

 ;in an early stage of development, being in the condition of neuroblasts 

 the cytoplasm of which has become drawn out to one side preparatory 

 •to developing into neuraxons. In the mesocephalic ganglion many 

 ganglion cells have already sent out their neuraxons, and these form 

 the trunk of the ophthalmic division of the fifth nerve. There are also 

 present, however, young ganglion cells which have not reached this 

 stage of development, as the one to which attention has just been 

 called (Fig. 5, y). In fact, such ganglion cells seem to be forming here 

 through the activities of proliferating cells, which can be seen in every 

 section made through the (Jasserian ganglion in this and preceding 

 stages. Since, then, the mesocephalic ganglion is the scene of constant 

 cell production, and since its young ganglion cells resemble exactly 

 those of the transitory ganglion, and are connected with them by a 

 continuous series of similar ganglion ^.ells lying scattered along the 

 nerve, the source of the component cells of the transitory ganglion does 

 not seem to be open to question, especially when we remember that 

 young ganglion cells possess an extraordinary capacity for locomotion 

 (His, Jun., '91). All the evidence points to a migration of ganglion 

 cells distally along the nerve to form the transitory ganglion and 

 similarly, any other smaller ganglia that may be found near it, 



