PLATE LXXVIII. 



FIGS. 184-186. Various stages In the transition of the primary into the secondary optic vesicle, and the 

 development of the lens at the end of the second and during the third day. 



186. o. Cavity of secondary optic vesicle, b. Rudiment of retina, c. Rudiment of pigment epithelium of the 

 choroid. d. A'crrin o/ttictu. e. Lens. /. Upper or corneal layer. 



184. a. Primary optic vesicle, and 6 its wall. c. Nervus opticut. d. Upper or corneal layer, e. Beginning of 

 lens. 



185. a. Primary optic vesicle. 5. Saucer shaped cavity, which subsequently becomes the secondary optic 

 vesicle, c. Ifervut opticut. d. Outer wall, and e inner wall, of primary optic vesicle, f. Upper or corneal layer. 

 g. Rudiment of lens. 



FIG. 188. Other forms of elements, in which blood corpuscles are produced, a, a, are the cavities of vesi- 

 cular structures, produced by the formation of vacuoles, in originally solid cells. The wall of the vesicle 6, 

 which consists of nucleated protoplasm, represents the endothclium of the future vessel, for which reason these 

 vesicles may be called endothelial vesicles. At d, blood corpuscles are detaching themselves from tin- inner 

 portion of a vesicle. /. Shows an element of another kind, iu which blood corpuscles are formed. It is a 

 spindle-shaped or branched solid cell, the central portion of which becomes blood corpuscles, and the peri- 

 pheral portion endothelium. ft. Is an element similar to that in fig. 187. 



These three varieties of formative elements of blood corpuscles are in communication with each other by 

 solid offshoots. They have this in common, that in all a peripheral layer of nucleated protoplasm is dif- 

 ferentiated from the interior, which contains a greater or less number of blood corpuscles. The interiors of 

 neighbouring elements eventually become continuous with each other by the offshoots or communicating 

 threads above mentioned, which become hollowed out, and thus give rise to a system of tubes, the blood-vessel*. 



