114 ARTHUR DENDY. 



ordinary paired eyes have begun to invaginate to form the 

 optic cups, while the lens already appears as a thickening of 

 the superficial epiblast. 



A short way behind the ordinary paired eyes (actually in 

 front, owing to the cerebral flexure) a small round vesicle 

 appears as a bud on the roof of the fore-brain, slightly to the 

 left of the middle line. Owing to the cerebral flexure it 

 appears in sections on the ventral surface, as shown in fig. 2. 

 This vesicle I propose to term the primary parietal 

 vesicle. In the section figured, the roof of the fore-brain, 

 with the primary parietal vesicle, is cut almost tangentially, 

 in a direction not favourable for studying its histological 

 characters, and I therefore reserve the description of these 

 until later. 



The position of the primary parietal vesicle to the left of the 

 middle line is very remarkable, and appears to be constant 

 (see also fig. 5, which is reversed as compared with fig. 2), 

 but the discussion of this problem must be postponed till the 

 sequel. 



Stage L. 



At Stage L, comparable with a chick of about three days, 

 the primary parietal vesicle has become conspicuous, when the 

 head of the embryo is examined in profile, as a small pro- 

 tuberance on the roof of the fore-brain between the paired 

 eyes. Longitudinal vertical sections of the head show it lying 

 a short way in front of the mid-brain (fig. 3). It still com- 

 municates by a wide aperture with the brain-cavity, but the 

 vesicle itself is somewhat flattened dorso-ventrally and extends 

 on both sides of this aperture, rather more forwards than 

 backwards (fig. 4), so that it appears to be attached to the 

 brain by a very short, hollow stem. Its wall is composed of 

 a layer of columnar cells similar to those forming the under- 

 lying wall of the brain, with large, oval, finely granular nuclei 

 and indistinct cell boundaries. The outer wall of the vesicle 

 lies immediately beneath the epidermis, and the mesoblast 

 cells are only just beginning to spread in between the two. 

 The cavity of the vesicle is filled with a finely reticulate 



