DEVELOPMENT OF THE CHICK 1 (1) 



1. Examine, with the dissecting microscope and with the low power of 

 the compound microscope, a permanently mounted three-day Chick 

 embryo. Note that the general shape resembles a 'reversed question- 

 mark' with the head end turned toward the right. Anteriorly, the em- 

 bryo is lying on its left side and, posteriorly, on its ventral surface. On 

 each side of the embryo, note the vascular area with the large vessels 

 (vitelline arteries and vitelline veins) which enter the embryo near 

 the middle of the body. 



2. Beginning at the posterior end of the body, identify the median 

 spinal cord, and on each side of it the segmental, paired, muscle plates 

 (myotomes). Ascertain the number in your specimen. Find the amnion 

 which later completely encloses the embryo, but at this stage covers only 

 the anterior portion of the embryo and ends somewhat posterior to the 

 region where the vitelline vessels enter the body. 



3. Study the anterior end of the embryo and locate: (a) the fore- 

 brain, with the rudiments of the eye and nose ; (6) the large, rather 

 spherical mid-brain, and, to the left, (c) the smaller hind-brain, with a 

 large, dorsal depression (fourth ventricle), (d) The rudiment of the 

 right ear is situated near the posterior end of the hind-brain, and appears 

 as a small, pear-shaped vesicle, (e) The twisted, S-shaped heart lies 

 outside the body wall in the space between the tip of the head and the 

 curved portion of the trunk. It consists at this stage of (/) a large poste- 

 rior ventricle, into which (g) the auricle opens. A portion of the 

 auricle can be seen to the left. Leading from the ventricle, anteriorly, 

 is (h) the conus arteriosus. It soon divides to form a number of bran- 

 chial vessels which run dorsally between (i) the gill slits to form a 

 portion of the aortic arches. 



4. Make a full page drawing of the entire embryo to show the structures 



as observed. 



i B. pp. 232-238. 



363 



