246 bulletin: museum of comparative zoology. 



and nuclei is still more sharply emphasized. I think we may safely 

 assume that this effect is the same as that resulting from a shoving of 

 the neural tube due to rapid growth in a confined space. Figure 28 

 (Plate 5) shows a frontal section of a preparation of a shark embryo, 

 nearly 10 mm. long. The specimen was fixed in the mixture of picro- 

 sulphuric and chromic acids, and then transferred directly to 50 per cent 

 alcohol. Inadequate fixation and immediate transference to a fluid of 

 very different osmotic power resulted in a strong contraction of the em- 

 bryo, particularly emphasized in the wall of the neural tube. (In the 

 figure the constrictions appear exaggerated, since only the regions of 

 the nuclei are shaded.) Moreover, a comparison of embryos of differ- 

 ent Vertebrates gives evidence, as it seems to me, that the bending of 

 the neural tube results in the intensification of the characteristics of 

 neuromeres. I have studied in frontal section embryos of Petromyzon, 

 Gadus, Amblystoma, S. acanthias, chick, and swine. The radial arrange- 

 ment of cells is more pronounced in those forms which have a stronger 

 flexure, and in which, therefore, we may safely assume that there is a 

 greater shoving of the neural tube, due to rapid growth in a confined 

 space. These characteristics are considerably more pronounced in Sau- 

 ropsida than in S. acanthias, in which the flexure of the neural tube is, 

 however, considerable. This explanation tends to remove the doubt as to 

 the phylogenetic value of such structures as the neuromeres which nat- 

 urally arises when these are shown to be structures slightlyif at all 

 visible in the lowest Vertebrates (Amphioxus and Cyclostomes), while well 

 marked in the highest. I believe that the presence of yolk makes the 

 conditions in both Petromyzon and Amblystoma less primitive than in 

 Squalus, chick, and swine. 



In Gadus and Amblystoma the radial arrangement of cells and nuclei 

 is even less pronounced than in S. acanthias, and this seems to be corre- 

 lated with the fact that the flexure of the neural tube in the former is 

 less marked than in the latter. It must be admitted, however, that the 

 presence of much yolk in the cells of the neural tube of Amblystoma 

 (Plate 5, Fig. 35), in which no sign of encephalomere IV is present, may 

 be concerned in producing the different condition of this form, in which 

 the outpocketing of the neural tube takes place in the region of the pro- 

 liferations of the ganglionic Anlagen only. Broman ('95, p. 186) has 

 given proof, satisfactory as it seems to me, that the nuclear and cellular 

 characteristics of the neuromeres of the human embryo may be explained 

 partly on mechanical grounds. Embryologists are agreed that the flex- 

 ures of the neural tube may be accounted for by the rapid growth of the 



