CARPENTEE: DEVELOPMENT OF THE OCULOMOTOR NERVE. 143 



included in this paper my observations on the histogenesis of tlie abdu- 

 cens. Its study has been made easy by its presence in the series of 

 sections in which the later development of the oculomotor has been fol- 

 lowed, and as a typical ventral cranial nerve with motor functions it has 

 proved interesting for the purposes of comparison with the oculomotor. 

 The remaining eye-muscle nerve, the trochlear, first appears at a com- 

 paratively late stage, and my observations have not been extended to it. 



Several considerations led to the selection of the chick as the subject 

 for investigation. First, closely connected stages in the development of 

 the eye-muscle nerves have been studied in but few of the Amniota, the 

 greater part of the observations having been confined to selachians. 

 Secondly, no investigator has directly concerned himself with the genesis 

 of these nerves in birds since Marshall published the first accounts of 

 their development in 1877 and 1878. Marshall's descriptions are ad- 

 mittedly incomplete, and certain of his interpretations are questionable 

 in the light of more recent studies in other classes of vertebrates. Since 

 his time observations on the nerves in question have been fragmentary 

 and incidental. Thirdly, in the case of the chick it is possible to control 

 incubation, and obtain embryos in the required stages of development. 



It is a pleasure to acknowledge here my sense of obligation to Professor 

 E. L. Mark, under whose guidance the present work was carried on in the 

 Zoological Laboratory of Harvard University. The constant interest, 

 the helpful suggestions and the conservative judgments of Professor 

 Mark have been of the greatest value to me. I am also indebted to 

 Professor H. V. Neal, o^ Knox College, for advice as to the use of the 

 vom Rath fluid in the early stages of the development of nerves. 



PART I. -ANATOMY AND HISTOLOGY. 

 A. Historical Survey. 



a. Anatomy, 



1. Eye-Muscle Nerves. 



The older anatomists (Muck, '15 ; Bonsdorff, '52 ; Budge, '55 ; and 

 others) who first investigated the eye-muscle nerves of birds found them 

 homologous with nerves of similar function in ocher classes of verte- 

 brates. The third, or oculomotor, nerve arises from the ventral face of 

 the mesencephalon, and is distributed to the dorsal, ventral and anterior 

 rectus muscles and to the ventral oblique muscle. The fourth, or troch- 



