6 GENERAL CONCEPTIONS. 



as an appendage to the septum itself, that the liver is developed, although it is 

 produced by a special local growth of the digestive canal. 



9. The urogenital ridge. Out of a part of the primitive segments there are 

 developed excretory organs, and these, as they increase in size, form two pro- 

 tuberances on the dorsal side of the splanchnocele. Each protuberance is what 

 we know as the urogenital ridge, so named, first, on account of its form; and, 

 secondly, on account of its producing not only the excretory organs proper, but 

 also the genital glands. 



10. The urogenital ducts. There is primitively a single duct for each uro- 

 genital ridge. This duct is commonly known as the Wolffian duct. A little later in 

 the history of the embryo there appears a second canal known as the Mullerian 

 duct, which is closely parallel to the first, but which has no connection with any of the 

 excretory apparatus, and is destined to serve later as the female genital duct. In no 

 invertebrate have we found anything certainly homologous with these two ducts. 



11. Special sense-organs. These are the olfactory, the visual, the so-called 

 auditory organs, and the organs of the lateral line. We have to use the term 

 "so-called" in speaking of the auditory organ because we now know that the ear 

 in the lower vertebrates is not an organ of hearing, but an organ of balancing or 

 orientation, and it is only in the higher vertebrates that there is added to this 

 primitive function that of audition proper. It seems not improbable that many 

 invertebrate animals have sense-organs which are homologous with those of verte- 

 brates. Nevertheless, in the vertebrate type there are many peculiarities which are 

 distinctive, and these we shall best learn from a 'study of the actual development. 

 The sense-organs of the lateral line are highly developed important structures in 

 the Ichthyopsida, but apparently are not represented in mammals at any stage 

 of their development. 



12. The hypophysis. The hypophysis is the embryological name applied to 

 the structure which we know in the adult as the anterior lobe of the pituitary 

 body. The posterior or infundibular lobe is a portion of the brain, but the an- 

 terior lobe is an outgrowth from the cavity of the mouth of the embryo. Com- 

 paratively early in the development of the individual this outgrowth becomes 

 entirely separated from the mouth-cavity (from the walls of which it arose), and 

 forms a closed vesicle. It exists in every known vertebrate animal, has been 

 much studied, but still remains an organ the significance of which we cannot ex- 

 plain. Its absolute persistency and the uniformity of its development indicate 

 that it is an organ of importance, but beyond that we can hardly go. 



To these conceptions, the student should add the following comprehensive 

 morphological notions: The mammalian body may be defined as two tubes of 

 epithelium, one inside the other; the outer tube (epidermal or ectodermal) is very 

 irregular in its form; the inner tube (entodermal) is much smaller in diameter, 

 but much longer than the outer and has a number of branches (lung, pancreas, 

 etc.), and is placed within the ectodermal tube. Between these two tubes is the 



