An Early Embryo of Myrmecophaga jubata. 261 
cavity. It reaches almost to the tip of the tail, and is clearly marked off at 
its hind end from both the spinal cord and post-anal gut. The spinal cord 
ends blindly and has a wide cavity throughout its length. The spinal nerves 
have both dorsal and ventral roots, and can easily be traced in a ventral 
direction to the level of the dorsal aorta. The spinal ganglia are recognisable 
almost to the tip of the tail. 
The medulla oblongata is large, with no trace of a cerebelium in front. 
Some of the cranial nerves are recognisable, namely, VII, VIII, IX and X 
with numerous roots. 
The otocyst is narrow from side to side and is slightly concave towards 
the brain. It has a wide blunt projection ventrally, and a long slender 
tube—the ductus endolymphaticus—with a dilatation at its root, passes 
forwards and upwards. It has no connection with the skin. 
The lentic vesicle is completely closed, and its walls are still equally 
thick all round. The retinal layer of the optic cup is much thicker than 
the other but there is no trace of retinal pigment. The choroid fissure is 
distinct, and the optic stalk has a wide cavity open to the brain. 
The infundibular evagination is short and curved, and in the hollow 
between it and the brain lies the pituitary body, which is open to the buccal 
cavity by a narrow tube. 
The olfactory organs are short pockets open to the exterior in the 
neighbourhood of the mouth. They have no connection with the buccal 
cavity at their inner end. A shallow groove on the inner side indicates the 
beginning of Jacobson’s organ. 
The pharynx in the region of the gill clefts is distinctly asymmetrical, 
the left side being further advanced in development. A slit extends 
backwards from the corner of the mouth on the left side for a considerable 
distance. It closely resembles a gill cleft at its lower end. 
The first two gill clefts are open on each side and the third gill pouches 
are formed, though they do not yet reach the exterior.. That on the right is 
very small. There is a slight invagination of the ectoderm in the position 
of the future cleft on the left side, which hes close to the opening of the 
second cleft. It is curious that Phisalix’s human embryo also shows a slit 
from the mouth on the left and an asymmetry of the gill pouches in which 
the left side is further developed. 
A small unbranched diverticulum near the mid ventral line at the level 
of the third gill pouch may represent the thyroid. 
On the left, below the second gill cleft, the ectoderm comes into 
close relation with one of the ganglia of the vagus, forming an organ 
of Froriep. 
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