SAGITTAL SECTIONS OF EMBRYO OF 12 MM. 205 



of equal size, but in this embryo the right vein, V . U. D, has already become 

 smaller than its fellow, V. U. S, of the left side, and in later stages the right vessel 

 is found to have disappeared altogether. The two veins are here connected 

 respectively with the right and left ventral lobes of the liver. It will be noticed 

 that the right and left sides of the abdominal cavity are completely separated 

 from one another, and that there is a special part of this cavity shown in the 

 section between the stomach and the right dorsal lobe of the liver, and which is 

 known as the lesser peritoneal space or cavity of the omentum. In another sec- 

 tion the lesser peritoneal space is found to connect, by means of a very small and 

 narrow foramen of Winslow, with the general cavity of the abdomen. 



The Study of Sagittal Sections, Embryo of 12 mm. 



From a sagittal series of this stage many significant pictures are obtainable. 

 Two sections only have been selected for illustration and description : first, the 

 median section of the head ; the second one passing through the principal ceph- 

 alic ganglia. To the student of anatomy these sections are highly instructive, 

 for they exhibit each, or in a single picture, many important fundamental rela- 

 tions of the brain, cephalic nerves, and other structures of the head. 



Median Sections of the Head. — The plane of the section (Fig. 123) is almost 

 exactly median for the region of the hypophysis and infundibular gland, but in 

 the region of the spinal cord it is a little to one side ; hence the actual plane of the 

 section is slightly oblique to the true median plane of the embryo. The present 

 section is especially instructive as regards the shape of the brain and the relations 

 of its various parts to one another. The hind-brain begins at the spinal cord, 

 Sp. c, and has a very large cavity, the fourth ventricle, Ven. IV. It is sepa- 

 rated from the region of the mid-brain by a constriction which is very marked on 

 the dorsal side, Isth. The constriction is known as the isthmus. It is always 

 from the dorsal side of the isthmus that the fourth nerve takes its origin. It is 

 one of the fixed landmarks of the brain. The mid-brain, M. B, also has a large 

 cavity, and, as a whole, forms the great arch which corresponds to the head-bend 

 of the embryo. It passes forward and downward, without any very definite line 

 of demarcation at this stage, into the fore-brain, the cavity of which is larger in 

 diameter than that of the mid-brain. The fore-brain is partially subdivided into 

 two regions; the anterior, Pros, is the prosencephalon and gives rise to the lat- 

 eral outgrowths, which form the cerebral hemispheres. Already the deep depres- 

 sion separates this part of the fore-brain on its dorsal side from the posterior 

 part, which is termed the diencephalon. The limits of the diencephalon at this 

 stage are very indistinct; later its boundary against the mid-brain becomes 

 clearly marked by the differentiation of the epiphysis and posterior commissure. 



