114 WALTEE HEAPE. 



further outwards, but do not at this stage fuse with the 

 external epiblast (fig. 21). 



The wall of the optic vesicles is similar in structure to the 

 wall of the remainder of the fore-brain. 



It is interesting to note that for a considerable period after 

 Stage J the optic vesicles show but very slight advancement on 

 the conditioQ then attained ; their growth appears now to be 

 retarded in as marked a degree as it was advanced in the early 

 stages. The early appearance of the optic grooves will probably 

 be recognised as a mammalian distinction when the embryo- 

 logy of more species of Mammalia has been worked, but the 

 sudden checking of the development in the Mole we may 

 expect is due to the specialisation of this species. Any modi- 

 fication of an important sensory organ would doubtless rapidly 

 afi"ect the development of the organ, but such an extended 

 modification as is apparent here says much for the primitive 

 nature of the habits of the animal. 



The Ear. — The first indication of the ear arises during Stage 

 H as a thickening of the external epithelium on each side the 

 hind-brain (fig. 25). The thickening extends along a great 

 portion of the hinder half of the hind-brain, and during Stage 

 J increases in thickness and becomes grooved along the greater 

 part of its length (fig. 46), 



The Cranial and Spinal Nerves. — I do not propose to describe 

 the development of the cranial and spinal nerves in this paper. 

 I hope to make a separate communication upon this portion 

 of the development at some future time. 



The Hypoblast. 



The hypoblast in the earliest condition of Stage e is similar 

 to what it was in Stage d (described in my former paper, No. 

 8), and is composed of a single layer of flattened cells extending 

 on all sides over the embryonic area (figs. 13, 14, and 15). 



The cells in the median line give rise to the notochord, and 

 the changes they undergo will be described in detail in another 

 section of this paper. 



The formation of the deep medullary groove in Stage d and 



