106 WALTER HEAPE. 



at each end. A shallow medullary groove runs down the centre 

 of the long axis of the embryo, which in its turn is narrow in 

 the centre and wider at either end. On each side the medul- 

 lary groove in the central narrow portion of the embryo, three 

 protovertebrae may be seen already formed. 



The hinder end of the embryo is thickened owing to the 

 growth of the mesoblast of the primitive streak, while anteriorly 

 it is flattened out to form the cephalic plate. The shaded 

 portion surrounding the embryo {a. p.) is the extent of the 

 area pellucida at this age. 



Fig. 2 represents a slightly older embryo of the same stage 

 of growth (r82 mm. long). The medullary folds have here 

 begun to form, they are raised somewhat, and in the centre of 

 the embryo are already approximated. At the anterior end the 

 floor of the medullary groove, on either side, is swollen, and on 

 the outer and anterior edge of the two masses so formed a deep 

 narrow groove indicates the commencement of the formation 

 of the optic organs and will be referred to as the " optic 

 grooves." 



This early appearance of the organ of sight is, so far as I am 

 awarCj peculiar, and is worthy of notice; even at this age the 

 grooves are directed outwards and downwards, and have their 

 origin from the most anterior portion of the medullary groove. 

 The curved condition of this embryo is due to careless manipu- 

 lation whilst it was in a fresh and soft state. 



Stage F. — Fig. 3 represents an embryo of this stage of 

 growth; it is 1"96 mm. long. The medullary folds have met, 

 although they have not yet coalesced, in the middle of the 

 embryo, and have extended thence forwards. 



The anterior end of the medullary canal is, however, still 

 widely open, and the two thickenings of the floor and sides of 

 this portion are shown. The optic grooves are also indicated 

 in the same manner as they were in the previous figure. 



It will be observed that the sides of the medullary canal at 

 the anterior end have grown forwards in advance of the floor. 

 At the hind end the medullary canal is widely open, forming 



