304 Natural History Bulletin. 



The photographs were made with Bausch & Lomb Student 

 objectives, in an Atwood camera. Ordinary 4x5 inch dry- 

 plates, made in Iowa Cit}', were used. They were developed 

 by the usual ferrous oxalate developer. 



OBSERVATIONS. 



The first studies were made on an embryo about five 

 millimeters in length. A series of longitudinal sections was 

 prepared in the manner described. Figure i is from a section 

 very near the median plane of this embryo. 



At this stage the embryo of the cat does not correspond 

 very closely to any particular hour or day in the development 

 of the chick. The circulatory system resembles in some 

 respects that of a chick of the fourth da}-, the heart being a 

 curiously twisted tube, five visceral arches being present. 

 The first aortic arch is broken; the second almost obliterated; 

 the third and fourth quite large and conspicuous; but the fifth 

 has not yet appeared, although the fourth visceral cleft is 

 present. 



The nervous system is somewhat like that of a chick of the 

 second or third da}^ the three primary cerebral vesicles being 

 present, the cranial flexure quite pronounced, and the cephalic 

 end of the embryo having much the appearance of a retort. 



In this embr3^o the primitive ocular vesicles have just made 

 their appearance, and the primitive otic vesicles are also form- 

 ing; but nothing has been observed that would indicate that 

 the vesicles of the cerebral hemispheres are begun. 



Figure 2 gives an enlarged view of the cephalic extremity 

 of the section from which figure i was taken. The meso- 

 blastic axis in this region is covered by the epiblast lining the 

 coeliae of the encephalon. Most of the mesoblastic cells are 

 branched, far apart, each with a large nucleus and a con- 

 spicuous nucleolus. The cells forming the long projection 

 between the anterior and posterior cerebral vesicles are ar- 

 ranged in curves, as if they had been compelled to arrange 

 themselves thus by the flexure of the cephalic end about this 



