334 ANATOMY OF THE DOMESTIC FOWL 



of divergence of the walls of the fore gut; also through the somatic 

 region, and through the primitive streak. 



Study all parts, as the head fold, the heart, and the fore gut. 



3. THE EMBRYO FORTY-FOUR TO FORTY-EIGHT HOURS OLD WITH 

 TWENTY-FOUR TO TWENTY-NINE SOMITES. 



Remove the embryo with the entire area, and preserve it. 



Note and carefully describe the changes visible to the naked eye 

 since the thirty-fourth hour. 



In making a study of the entire embryo we note there has been a 

 rapid growth of the dorsal surface of the head, which has become 

 more bent. This bend, in the region of the midbrain is called the 

 cephalic flexure. The forebrain and part of the midbrain form 

 almost a right angle with the rest of the head. The head is com- 

 pressed laterally and free from the blastoderm. The dorsal side 

 of the trunk is turned up, and there is a twisting of the axis of the 

 embryo just back of the heart. The tail fold begins at about this 

 time and may or may not be visible. The optic vesicles are rela- 

 tively smaller in relation to the brain than in i. Note the part 

 of the forebrain to which they are attached. Observe the inner and 

 outer layers of the retina, the lens, the choroid fissure, and the 

 cavity of the vitreous humor. 



Note the auditory vesicles and whether or not they are closed sacs. 



Note that the heart has grown in length and has become doubled 

 on itself. The two ends are fixed. Note the relation of the heart 

 to the afferent and efferent blood-vessels. Note that two, and 

 possibly three, visceral pounches are visible. Note whether or not 

 they are ventral to the midbrain. The first, the hyomandibular 

 pouch, is bounded in front by the first visceral, or mandibular, and 

 behind by the second visceral, arch. The second pouch is bound in 

 front by the hyoid, and behind by the third visceral arch. The 

 third pouch is bounded in front by the third visceral arch, and 

 behind by the fourth. Note the number of mesoblastic somites and 

 the condition of the mesoblastic segmental plates. 



Note how far back the foregut is closed. Locate the head fold of 

 the amnion, and note how far back it is closed. Note changes that 

 have taken place in the vascular area. In studying the sections it 

 will be found that a section cut transversely to the trunk will pass 

 horizontally through the forebrain and through the midbrain. 



Study a section through the trunk a short distance behind the 

 heart. Observe the elevation of the axis of the body. Note the 



