180 STUDY OF YOUNG CHICK EMBRYOS. 



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 vascu- 

 losa, but not yet in the embryo. It will be seen, therefore, that though the pro- 

 portions differ greatly from those in the chick, the fundamental relations in the 

 rabbit are the same as in the bird. 



Examination of the Specimen after Staining. After the chick has been stained 

 in toto it should be cleared in oil of cloves, or other suitable fluid, and further 

 examined in surface views with low powers of the microscope. For this purpose 

 it may be placed in a small shallow staining dish. It will be found advantageous 

 to have also whole chicks with their area? vasculosae permanently mounted in 

 Damar. Embryos up to about forty-eight hours' incubation are readily prepared in 

 this way. The germinative area with the embryo is treated like an ordinary section. 

 Specimens thus mounted must be protected from the pressure of the cover-glass 

 by putting under two opposite edges strips of paper or, better, of glass to support 

 the cover. Strips of glass as needed can be cut from broken cover-glasses with 

 a writing diamond. 



Figure 131 represents a chick with eight fully formed segments stained with 

 alum carmine and viewed as a transparent object. 



The distribution of the blood-islands and various details of the structure of the 

 embryo, which in the fresh specimen are obscure, can be readily observed in the 

 cleared preparation. The blood-islands, Bl.is, contain crowded young blood-cells 

 and are conspicuous owing to the intensity with which they are stained. They are 

 largest and most numerous in the posterior part of the area opaca, and on either 

 side become gradually smaller and farther apart toward the anterior end and are 

 absent altogether at the level of the head. A few small islands appear in the area 

 pellucida around the caudal end of the embryo. The amnio-cardiac vesicles, A.c.v, 

 are marked by the arching up of the surface of the area pellucida on both sides 

 of the head. In the embryo, the segments, Som.$, the walls of the medullary tube 

 (brain, Br, and spinal cord, Sp.c), and the omphalo-mesaraic veins, V.om, are 

 sharply defined. The first segment is imperfectly formed, and never acquires a 

 full development; it, together with segments two, three, and four, are called the 

 occipital segments because they enter into the formation of the occipital region of 

 the head and never undergo full differentiation like the other segments. In mam- 

 mals also there are (probably four) occipital segments. The fifth segment of our 

 chick becomes the first cervical segment of the adult. The medullary tube, Md, 

 has sharply defined, walls. It is completely closed through the brain region, except 

 at the anterior neuropore. At its cephalad extremity the tube has expanded later- 

 ally to form the optic vesicles, Op.V, each of which is the anlage of a retina and 

 optic nerve. The middle region of the tube, between the optic vesicles, is the first 



