54 MORGAN. [Vot. IX. 
and relatively much broader than in the first figure. It will 
be remembered that the nerve cord is cut off from the ecto- 
derm in this condition. 
We must conclude that the nerve cord, to keep pace with the 
growing collar, has elongated at the expense of tts breadth. 
Serial sections of the nerve cord at the stage of three gill- 
slits (a little later than Fig. 70), give the structure shown in 
Fig. 73. The cord has now sunken far beneath the surface 
of the ectoderm, and is connected with the latter by a band of 
tissue formed by the apposition of the walls of the collar body- 
cavities. No ectodermal cells can be found between the layers 
of mesoderm. The nerve cord has a relatively large lumen, 
but the most conspicuous feature is the scarcity of nuclei 
in such sections. On an average, about twelve are to be 
found. This fact, taken in connection with the extension of 
the nerve cord, as seen in surface view, leads to the con- 
clusion that the length is obtained at the expense of the tissue 
already present, this being simply pulled out. We will return 
to this point again for special consideration. 
Beneath the nerve cord, there is in the figure a space 
bounded on the outside by the collar body-cavities (0.c.2), but 
having within these walls the forward extensions of the last 
pair of body-cavities (0.c.’). In this way the cavity of the dorsal 
blood-vessel of the trunk is carried forward between the pro- 
longations of the last body cavity (6.c.*). 
In the lower part of the section are the much vacuolated 
cells of the dorsal wall of the digestive tract. 
The forward extensions of the last pair of body-cavities, seen 
in the last section, reach to the anterior end of the collar where 
they are succeeded by a similar forward prolongation of the 
collar-cavities and the latter extend as far forward as the 
proboscis vesicle. 
The notochord extends farther forward and into the region 
ventral to the proboscis vesicle, 2. ¢., into the region where the 
so-called heart was found in earlier stages. The cavity of the 
heart is not entirely obliterated, but is to be found above as 
well as anterior to the notochord. It is along the dorsal sur- 
face of the notochord that the collar-cavities reach the proboscis 
