EMBRYO WITH EIGHT SEGMENTS. 



179 



This will enable the student to make out the blood-islands and something of the 

 blood-vessels in the area vasculosa, and also the shape of the brain which (Fig. 

 131) has expanded widely just behind the neuropore; the lateral expansions are 

 the anlages of the optic vesicles (Fig. 133). The remainder of the brain extends 

 from the optic enlargement to a point a little behind the fovea, fov. It is much 

 wider than the remaining portion of the medullary canal; it tapers from the optic 



FIG. 130. EMBRYONIC AREA OF A RABBIT WITH EIGHT SEGMENTS, WITH THE PLACENTAL AREA PARTLY TORN OFF. 



X 15 diams. (Drawn by T. H. Emerton.) 



vesicle and extends backward. One cannot yet distinguish in it positively any 

 subdivision into mid-brain and hind-brain. On the contrary, its walls are often 

 somewhat irregularly sinuous and vary considerably from specimen to specimen. 



Comparison with a Rabbit Embryo. In the ovum of the mammalia the ecto- 

 derm presents a modification known as the trophoderm. In the rabbit this tropho- 

 derm is developed over a limited region which is called the placental area (Fig. 

 130, a. pi), by which the embryo is attached to the wall of the uterus. When 

 the embryo figured was removed, a portion of the placental area remained attached 

 to the uterus, hence the defect shown in the specimen. The vascular area is 



