926 THE AMERICAN NATURALIST. (VoL. XXXIV. 
the first and larger is the section of the cervical, pharyngeal 
and mandibular regions; the second and smaller is the section 
from the tip of the low bent head. The spinal cord (.S7.c.) in 
the upper part shows clearly both the three primary layers of 
its walls and the division into dorsal and ventral zones. The 
outermost layer is very lightly stained, and may be, perhaps, 
best defined as the anlage of the white matter. The inner 
layer is deeply stained, while the middle layer (mantle or gray 
stratum) is intermediate in color. The nerve cells develop 
most abundantly and rapidly in the ventral part of the mantle 
layer, which is consequently more enlarged than the dorsal 
part and has caused a bulging sidewards and downwards of the 
ventral zone. The ganglion (Cerv.2) is the second cervical; it 
sends a root upward to enter the dorsal zone of the spinal cord, 
and a tract downward to join the ventral root, thus constituting 
the second spinal nerve (/Vv.2), which subdivides almost at once 
into a dorsal and a ventral ramus. Between the dorsal sum- 
mit of the ganglion and the spinal cord there is a small bundle 
of nerve fibres, not shown in the figure. These fibres consti- 
tute the commissural trunk of the eleventh nerve. The third 
gill-cleft (Z77.) is cut almost symmetrically, and extends from 
the median line to the edge of the section; it is lined through- 
out by the entoderm, which at the end of the cleft on each 
side has met and fused with the ectoderm to form an epithelial 
membrane (Verschlussplatte), which closes the cleft laterally. 
All vertebrate embryos probably have their gill-clefts at an 
early stage all closed by a similar membrane ; but, whereas in 
fishes and amphibians the membrane is soon broken through, 
in mammals, on the contrary, it remains intact, and the clefts 
are, it is thought, normally always imperforate. At the end of 
the cleft the entoderm has undergone a special growth, form- 
ing a distinct mass ( Thm.) on the side of the cleft towards the 
head. This entodermal structure is the anlage of the thy- 
mus, and is already penetrated by small blood vessels, which - 
are perhaps not capillaries but sinusoids. The third gill-cleft 
extends to the bottom of the cervical sinus, a large and deep 
depression of the outer surface, clearly shown both in Figs. ! 
and 5. The sinus is bounded cephalad by the large and 
