15 



If we examine the embryo at the commencement of the 

 second day, we see further progress in development. 



The head end of the neural canal is enlarged, and 

 forms the commencement of the brain on either side of it, 

 in front two large lobes appear, the optic lobes; and behind 

 these lobes two depressions, the auditory pits. The pro- 

 tovertebras of which there were only a few at the head end 

 of the embryo are increasing in number, and extending 

 towards the tail end. 



At this period the heart is really a part of the head, and 

 lies outside the body, the body wall not being yet closed, 

 and consists of a simple twisted tube, and in the living 

 embryo you can see, with the naked eye, the regular pul- 

 sations of this simple heart which pumps the blood to all 

 parts of the embryo, and over the blastodermic area. 



Even at this period can be discerned the bifucation of 

 the aorta, the main branch from the heart. 



The development of the eye. — You have already seen how, 

 at the commencement of the second day, the optic lobes 

 are a prominent feature at the head end of the embyro. 



This optic lobe is formed by the pushing out of the 

 layers of cells by the optic vesicle, a process of the brain. 

 The epiblast over the optic vesicle pits and this involution 

 meets it ; the pit becomes closed over, and the part thus 

 separated is the lens, and the epiblast which closes over the 

 pit becomes the cornea or external part of the eye. The 

 anterior surface of the involution of the brain, the optic 

 vesicle, which meets the lens becomes the retina, and the 

 posterior surface, the pigmental layer of the choroid. By 

 the walls of the vesicle coming into contact, and together 

 forming a cup, the cavity of the involution from the brain is 

 lost, and all that remains to show its connection with the 

 brain is a stalk which becomes the optic nerve. One part 

 of the retina does not close in, so that a fissure is left called 

 the choroidal fissure, and through this fissure the mesoblast 

 creeps in, and forms the vitreous humour, and the external 

 and internal muscles of the eye. 



