PRIMAEY NEUEOMERES AND HEAD SEGMENTATION 343 



In the lateral ridges a beaded appearance is sometimes present, 

 but in no case did it show the regularity described by Locy. The 

 lobulations along the neural crests are often entirelj^ absent, and, 

 when present they do not show definite regularity, either in 

 size or arrangement. The number of lobes varies from two or 

 three to as many as fifteen on one side, and little if any corres- 

 pondence could be detected between the lobulations of opposite 

 sides of the same embryo. Sections of the crests show the cells 

 to be distributed uniformlj^ with occasional slight irregular 

 groupings, but there is no evidence of a segmental arrangement. 

 These aggregates or clusters of embryonic cells are not differen- 

 tiated into regular areas, but appear to be centers of rapid cell 

 proliferation. Before the neural folds close the forebrain and 

 midbrain are clearly outlined by thickened enlargements, and 

 by the time of complete fusion, the three primary brain vesicles 

 are distinctly defined. The crests close rapidly and in essential 

 respects these observations confirm the description given by 

 Eycleshymer ('95). The median divisions disappear with the 

 closure of the neural folds, and no definite relation between them 

 and the brain vesicles could be determined. Sections of many 

 embryos seem to indicate that their fate is not uniform, but 

 differs in different individuals. 



For the study of chick embryos, several hundred eggs were 

 incubated, and over one hundred embryos were obtained for 

 study, giving a series of stages from the formation of the primi- 

 tive streak to the formation of the brain and spinal cord. Most 

 of the embryos were removed at the stage when the neural groove 

 is open, as this, according to Locy and Hill, is the most favorable 

 period at which to observe the primary neuromerism of the 

 nervous system. In the study of the living embryo most of the 

 work was done with a binocular although both dissecting and 

 compound microscopes were used. For illumination, trans- 

 mitted light, as well as reflected light from an electric arc, a gas 

 light, and also direct sunlight were used. Following Locy's 

 suggestion that "a dead black background is of course the best 

 surface for observing anything of this kind by reflected light," 

 a circle of dead black paper was placed under the specimen in the 



