128 CHARLES EUGENE JOHNSON 



he shows a dorsal and a ventral thickening of the wall of the 

 first head somite. From conditions shown by a much older 

 embryo, figure 32, he concludes that the dorsal gives rise to the 

 M. rectus superior and that the ventral one is the common anlage 

 of the Mm. obliquus inferior, rectus inferior, and rectus medialis. 

 The M. obliquus superior he derives from the dorsal part of the 

 second head somite. 



The abducent musculature is treated by him as one mass 

 from the time of its appearance as a single heap of cells repre- 

 senting the third head somite, up to the advanced stage shown 

 in figure 32, when, he states, a 'Zweiteilung' has taken place,' 

 with a corresponding forking of the abducent nerve. 



DESCRIPTIVE PART: MATERIAL AND METHODS 



The following investigation of the head somites and eye mus- 

 cles in Chelydra grew out of a study of the mesodermic somites 

 of the trunk region undertaken at the suggestion of Dr. B. M. 

 Allen, while doing graduate work at the University of Wisconsin, 

 in the summer of 1910. In continuing this study after returning 

 to Minnesota I became interested in the somites of the head and 

 began a more thorough study of them with the following paper 

 as a result. 



A considerable part of the work on this problem including the 

 making of the wax models was done during the past summer in 

 the Laboratory of Comparative Anatomy of the Harvard Medi- 

 cal School. The models were made under the guidance of Dr. 

 Frederic T. Lewis, to whom I am indebted for many favors, 

 helpful suggestions and never failing interest. It is also a great 

 pleasure here to acknowledge the kind interest and encourage- 

 ment of Dr. Mi not, who generously placed at my disposal the 

 entire Reptilian series of the Harvard Embryological Collection, 

 which was found invaluable in checking up and verifying a num- 

 ber of uncertain points in my own series, and for making a number 

 of instructive comparisons. To Drs. Minot and Lewis jointly I 

 am indebted for obtaining the services of Mr. William T. Oliver, 

 of Lynn, Massachusetts, by whom all the drawing of the wax 



