306 PROCEEDINGS OF THE ACADEMY OF [1884. 



an early stage of the embryology of a vertebrate, the anterior 

 end of the medullary groove, or canal, as the case may 

 be, is divided into three segments, which later form the brain. 

 The anterior of these is known by the name of fore-brain, or 

 proencephalon ; the middle one, or mid-brain, is called mesen- 

 cephalon ; and the posterior, the hind-brain, or metencephalon. 



From the fore-brain proceeds outwards and laterally a swelling, 

 which increases in size, and passes on to the epidermis. Here an 

 invagination takes place, inward, to meet this outward brain- 

 growth. This invagination finally closes, and soon becomes cut 

 off, to form a hollow vesicle, the cavity of which is finally 

 obliterated, and, becoming transparent, forms the lens of the 

 adult eye. In the meantime, the growth from the brain has 

 arched over and above this vesicle, and then folds over laterally 

 to enclose the lens. This process of the brain is hollow, and 

 communicates with the ventricular cavities of the brain. ^ This 

 differentiation which has taken place, to 

 form the so-called " secondary optic vesi- 

 cle," is hardly an invagination in the true 

 sense of the word, but is rather a double- 

 walled plate, which folds downwards 

 around the lens, this is indicated in the 

 i-iG. 4.— Diagram to iiius- diagrammatic representation in fig. 4. 



trate the method by which ™, , „,, ^, , . . „ 



the second»ry optic vesicle The ICUS fills up the anterior Opening 01 

 encloses the lens which , , . « , i i , . . , 



should fill up the open end. the cavity 01 the Secondary optic vesicle, 



yeo ver e ra e. ^^^ ^^ ^j^^ ^^^ edgCS, a and h, close 



around the under surface of the lens, a certain amount of meso- 

 dermic tissue is included, which later forms the transparent 

 corpus vitreum. After the closure is completed, there is a 

 double-walled vesicle, the interior wall is the thicker of the two, 

 and later gives rise to the many-layered retina; the external wall 

 forms the pigment layer of the chordidea. It not unfrequently 

 happens that we find incomplete closure of the secondary optic 

 vesicle, and when this is the. case in the adult eye, the patho- 

 logical condition known to physicians as coloboma exists. This 

 may take place in the iris (coloboma iridis^, or in the retina 



' It must be borne in mind that the interior of the lens was once a part 

 of the general surface of the body, and also the interior of the secondary 

 optic vesicle, proceeding first by the formation of the medullary groove, 

 and then from that inward. 



