108 BULLETIN OF THE 
vertebra shown in Figure 27 (Plate IV.), the two pairs of nuclei in 
the upper region bounding the spinal cord, which have just undergone 
division (n/."), are destined to increase the periphery of the protover- 
tebra ; and in Figure 17 (Plate III.) is an example of the proliferation 
of cells in the interior of the protovertebra. 
These three examples are the same in character, and tend to the same 
general purpose, viz. the enlargement of the protovertebra itself; but 
there is also a cell proliferation going on in its ventral region which has 
a purpose foreign to the augmentation of the protovertebra, for here 
the histological elements experience a very different fate. In the ventral 
region near the chorda a rapid cell proliferation (Plate ILI. Fig. 18, n?) 
is going on. Some of the cells which originate here pass ventrad, and 
take a position immediately under the chorda (nl.*), and I believe it 
very probable that they are concerned in the formation of the aorta. 
In this stage the lateral mesodermic layers are more distinctly sepa- 
rated from each other, leaving between them a relatively larger body 
cavity (compare Fig. 17, Plate III., and Fig. 13, Plate II.), and in the 
anterior region of the trunk they are undergoing rapid development. 
The process of shifting toward the median plane along the ventral side 
of the protovertebr&, the beginning of which has already been noted 
(p. 107), has perceptibly advanced. This process does not begin in the 
pectoral region and proceed backward, as might be inferred ; but is at 
first more pronounced back of the pectoral region, and only later takes 
place as far forward as this. This fact will be evident upon a glance at 
the series of drawings from a younger stage (Plate IL. Figs. 11-15). 
In an embryo of seventy-one hours the head-mesoderm (Plate IV. 
Fig. 23) in the region of the auditory vesicle consists of only a few cells, 
whose nuclei do not assume any definite arrangement. This condition 
is probably due, in part, to pressure upon the original mass of meso- 
dermie cells by the ectoderm and endoderm. The latter, constituting 
the beginning of the gill-clefts (fs. brn., Fig. 23), presses upward, while the 
former, constituting the beginning of the auditory vesicle (vs. au., Fig. 23), 
presses downward, thus confining the mesodermic cells within very 
narrow limits. The nuclei of the head-mesoderm are, however, readily 
distinguished from the surrounding ones of ectodermic and endodermic 
origin. Inu the region of the gill-clefts the head-mesoderm is completely 
cut off from the part which represents the somatopleure and splanchno- 
pleure (Fig. 23), and the latter (so’plu., sol plu.) are relatively thin lay- 
ers, bounding the pericardial cavity. In this stage of dovelopment the 
distal end of the gill-cleft (fs. brn.) has apparently nearly broken through 
