APPENDIX VIII. 331 



dens. The choroid cleft is just formed. The prosenceph- 

 alon, with its prolongation towards the extremity of the 

 snout ; the mesencephalon, which is hollowed completely 

 into the form of a cavity ; and the epencephalon, with the 

 cerebellum, which are beginning to form, are now very 

 easily distinguished. In front of the extremity of the cord, 

 which is still homogeneous, is accumulated at the base of 

 the brain the thick blastema of the basis of the cranium. 

 Then under the dorsal cord, between it and the yolk, forms 

 a thick layer of cells larger than the properly called em- 

 bryonic cells, and provided with opaque nuclei, the layer 

 of intestinal cells representing the mucous leaflet. This 

 layer is divided into two rows, the lower one designed to 

 form the intestine, the upper designed for the corpiis 

 Wolfianum. The intestine begins to be transformed into 

 a tube behind and in front, in proportion as the embryo 

 disengages itself more and more from the yolk. An 

 enlargement (a posterior allantois) shows itself at the ex- 

 tremity of the secretory canal of the corpus Wolfianum. 

 The heart forms in a swelling of the embryonic mass on 

 the side of the yolk, in the middle of the space between 

 the ear and the eye. At first solid, and composed of 

 simple embryonic cells, it is soon transformed into a cav- 

 ity, in which globules of blood can be seen to rise and 

 fall in cadence, conforming to the repeated contractions 

 of this organ. The heart is at a right angle with the axis 

 of the body, and reposes vertically upon the yolk, the 

 middle of which it occupies. Behind the heart can be re- 

 marked a little angular protuberance, the first vestige of 

 the pectoral fin. The blood-producing layer is seen to 

 appear upon the yolk in the neighborhood of the heart, 

 giving to the yolk a spotted appearance. The first traces 

 of the black pigment show in the choroid ; the cells of 

 brown pigment are created at the same time in the vicinity 

 of the eye. The vertebral divisions are very distinct. 



