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. Jong. 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 slightly if 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 $. 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 buman 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 
