CHICK, 44 TO 48 hours' INCUBATION 23 



Between the hind-brain and the heart in the region of 

 the phar}Tix are two or three pouches or clefts. These 

 are invaginations from the outside ectoderm. Immedi- 

 ately beneath each pouch is an evagination from the 

 phar^Tix. These are the gill-slits (visceral clefts or 

 pouches). Each cleft is bounded on either side by a 

 thicker mass. These masses are the arches. They 

 number from the anterior end. The first arch is the 

 mandibular arch. The second arch is the hyoid arch. 

 The hyomandihidar cleft is between the mandibular arch 

 and the hyoid arch. It is the first \'isceral cleft. The 

 other clefts and arches are numbered in order. Ventral 

 to the mandibular arch and beneath the optic vesicle 

 is a pit or invagination from the outside ectoderm. This 

 marks the location of the future mouth and is called the 

 oral pit. 



How many mesoblastic somites are there? What 

 changes have taken place in the lateral plates? In the 

 primitive streak? WTiere can you see the notochord? 



Draw the forty-eight-hour whole mount and label all the 

 parts. 



b. Transverse Sections 



The region from which is taken the cross section of 

 which you make a dramng, should always be located on 

 your drawing of the whole mount. 



( i) Through the Optic Vesicle showing the Lens, the Choroid Fissure, 

 and the Connection between the Optic Vesicle and the Fore-brain (the 

 Optic Stalk) 



Because of the cephahc flexure, the plane of this section 



