EMBRYO WITH SEVEN SEGMENTS. 297 



appearance extends forward and merges into the so-called segmental zone, out 

 of which the segments are differentiated. More careful examination of the 

 area opaca shows that it already possesses a well-defined area vasculosa, Ao, in 

 which (with some difficulty in the fresh specimen) traces of the formation of 

 blood-vessels and blood-islands can be made out. They can be somewhat better 

 distinguished if the fresh specimen be examined not by transmitted, but by re- 

 flected light. In the fresh specimen it is very difficult to make out the shape of 

 the medullary canal in the region of the head. 



Examination of the Specimen after Hardening. — The specimen, after it has 

 been hardened , should be examined under the microscope in water or alcohol ; and, 

 again, after it has been stained it should be cleared in oil and further examined. 

 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. 

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

 the anlages of the optic vesicles (Fig. 1 56) . 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 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 

 ectoderm presents a modification known as the trophoblast. In the rabbit 

 this trophoblast is developed over a limited region which is called the placental 

 area (Fig. 168, 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 re- 

 mained attached to the uterus, hence the defect shown in the specimen. The 

 vascular area is nearly circular; its boundary is marked by a well-defined 

 terminal vessel, v.t. The nearly straight embryo lies in the center and exhibits 

 plainly the medullary canal and primitive segments. The optic evaginations 

 are already present. The head is free; on its under side the heart is forming, 

 and beneath it is a relatively large and conspicuous pro-amnion, pr. a. Blood- 

 vessels are present over the area vasculosa, but not yet in the embryo. It 

 will be seen, therefore, that though the proportions differ greatly from those 

 in the chick, the fundamental relations in the rabbit are the same as in the 

 bird. 



Longitudinal Section of a Cluck. — In order to facilitate the study of the 

 transverse sections of this stage, figure 169 is inserted, which is a nearly median 

 longitudinal section. In consequence of the head end, H, having grown for- 



