68 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



Draw the cross-section. Do not attempt to put in individual cells, as these 

 are not distinctly visible. 



6. Later development and segmentation. Examine with the hand lens 

 embryos of about the stages of Holmes's Fig. 24 (p. 101) and Fig. 30, 3 (p. 117). 

 Note the elongation of the body with later appearance of a tail. In the younger 

 embryo observe that the nervous system is sharply marked off along the whole 

 dorsal side and that grooves, later to break through as gill slits, are present on 

 the sides of the head. In the older embryo identify the eyes on the sides of the 

 head, the ventral mouth with horny lips, the two suckers posterior to the mouth, 

 and the much-coiled intestine visible through the skin. Note especially in the 

 tail the zigzag segments. Each such segment is a unit of structure and will give 

 rise to a vertebra, a section of the spinal cord with a pair of spinal nerves, a 

 certain number of muscles, paired branches of the chief blood vessels, etc. These 

 segments are also present in the body region of the tadpole, although externally 

 invisible. Segmentation, that is, repetition of structures along the axis, in 

 fact underlies the whole make-up of the adult frog. 



