HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 129 



models were made, and also for valuable criticism and sugges- 

 tion in the final preparation of the manuscript. 



The material used in this study was obtained by the aid of 

 Prof. H. F. Nachtrieb, from the Embryological Supply Station 

 of Mr. Albert Allen, Madison, Wisconsin. All the material had 

 been faultlessly fixed and preserved. The following fixing agents 

 were represented: Tellyesniczky's bichromate-acetic; sublimate- 

 acetic; and Zenker's fluid. The embryos had all been preserved 

 in 80 per cent alcohol. 



The specimens prepared for study were stained in toto in 

 Meyer's hemalum for periods varying from twenty-four to thirty- 

 six hours. With the exception of two early series which were 

 not counterstained all the remaining were stained on the slide 

 in eosin. Sagittal sections proved by far the most satisfactory 

 for the study undertaken. But these were in a number of cases 

 supplemented by cross sections of corresponding stages. A num- 

 ber of temporary wax reconstructions were made from time to 

 time, of various structures, to aid in determining their form and 

 relation to other parts. 



THE HEAD SOMITES 



The youngest Chelydra embryo studied in the preparation 

 of this paper was a 2-mm. specimen with five segments. In the 

 prootic region of the head the dorsal mesoderm has a compact 

 uniform appearance on each side, becoming less dense towards 

 its anterior limits. A careful scrutiny of the district, however, 

 failed to reveal any differentiation of the mesoderm which might 

 indicate a possible somite area. The next older specimen was 

 a 3.5-mm. embryo 'with ten segments, in which, as the following 

 description will show, the mesoderm of the head presents differ- 

 entiations, the earUest phases of which undoubtedly may be 

 observed in stages lying between the 'two here mentioned. 



3.5-mm. embryo {10 segments); transverse series: figures 1 to S 



In this embryo the neural tube is still open at the anterior 

 end. Due to the flexure of the tube the plane of section is hori- 



