22 Charles Russell Bardeen 



Central Nervous System — The olfactory lobes extend between 

 the nasal organs. They are beginning to be fused in the region 

 shown in section a. Anterior to this they are more clearly separ- 

 ated and posterior to it the lobe on each side is continued into a well 

 marked cerebral hemisphere. Section b passes through the brain 

 slightly anterior to the junction of the two lateral with the third 

 ventricle. The anterior end of the choroid plexus is shown between 

 the two lateral ventricles and above this the posterior portion of the 

 pineal body. The latter extends forward for a considerable dis- 

 tance beneath the ectoderm. At this period it contains a small 

 vesicular cavity. The lateral walls of the third ventricle are thick 

 (section c). The optic stalk has been converted into an optic nerve 

 but has a short patent lumen near the third ventricle. This does 

 not appear at the level of section c. The infundibulum is large. 

 The opening into it from the third ventricle is small, but beyond here 

 it rapidly expands (section d). The optic lobes project dorsally 

 on each side of the midbrain (section d). The cerebellum is 

 marked by a slight thickening of the anterior margin of the roof 

 of the fourth ventricle. The ventro-lateral walls of the medulla 

 are very thick (section e). The folds of the choroid plexus are 

 beginning to appear in its thin roof. The spinal cord is well 

 developed and extends far back in the tail (sections /, g and 

 h). The Randschleier, in the region of the trunk, is as thick as 

 the layer of cells surrounding the central canal. 



Organs of Special Sense — Large nasal fossae extend from the 

 exterior to the pharynx. The epithelium of the medial wall of 

 each fossa is very thick (section a). Caecal outgrowths are taking 

 place from the posterior dorsal portion of each fossa. Bundles of 

 nerve fibers may be traced from the lateral side of each olfactory 

 lobe to the thickened epithelium of the corresponding nasal fossa. 



The eyes (section c) are well developed. The several layers of 

 the sensory part of the retina may be distinguished. A thin 

 sclerotic coat is present. The well formed lens is separated by a 

 distinct interval from the ectoderm. 



The auditory vesicles (section e) are being divided by the 

 ingrowth of septa into the various portions characteristic of the 

 adult ear. The auditory nerve is undergoing rapid development. 



