206 MANUAL OF ANIMAL BIOLOGY 



ovaries and the oviducts as in the case of the testes and the vasa 

 efferentia. 



Each of the coiled, tubular oviducts ends anteriorly in a ciliated, 

 funnel-shaped opening. These openings lie in the anterior end of 

 the coelom, one on either side of, and a considerable distance 

 anterior to, the ovaries. The action of the cilia in the funnels is 

 such as to cause a current to flow into them, and thus the eggs 

 are drawn into the oviducts. At the posterior end of each oviduct 

 before it opens into the cloaca there is an enlarged portion, known 

 as the uterus, in which the eggs collect, after passing down the 

 oviduct, and remain for a time before being discharged from the 

 body. During their passage through the oviducts the eggs are 

 coated with several layers of jelly which are secreted by glands 

 in the walls of the oviducts. The fertilization of the gametes and 

 the development of the zygote will be considered in the later section 

 on Vertebrate Development. (W. f. 132.) 



6. Endocrine System 



In the Frog and other Vertebrates there are a number of very 

 important glandular bodies, variously known as ductless glands, 



ORGANS OF INTERNAL SECRETION, Or ENDOCRINE ORGANS. Struc- 

 turally they are all characterized by the absence of ducts, so that 

 the secretions which they manufacture, known as hormones, or 

 internal secretions, are passed directly into the blood stream 

 which carries them to all the tissues of the body. The cells of the 

 various tissues select from the blood the specific hormone which 

 is essential to their welfare. Remarkable results have been ob- 

 tained within the last few years by the investigations upon the 

 various endocr'ne organs, but even so our knowledge of them and 

 of their physiological actions is limited. This much is clear, 

 however, that the ductless glands — undoubtedly controlled in 

 the final analysis by the nervous system — exercise through their 

 secretions, the hormones, a considerable coordinating and control- 

 ling influence upon various tissues and organs — a function which 

 is generally known as chemical coordination. Some of the more 

 important of the ductless glands may now be noted. (W. pp. 

 196-198.) 



Thyroid Glands. These consist, in the Frog, of a pair of tiny, 

 spherical bodies of a yellowish color, situated in the throat region 

 quite close to the larynx. They are richly supplied with vascular 



