THE VERTEBRATE TYPE OF STRUCTURE. 21 



7. The stomach, intestine, and mesentery. The division of the digestive 

 tract of vertebrates into these two fundamental parts is very characteristic. 

 The stomach is not only an enlargement of the digestive canal, but also may be 

 distinguished from the intestine by its developing glands, which are specific to it 

 and unlike those of the intestine proper. The mesentery by which the intestine 

 is suspended to the dorsal wall of the abdomen is the survival of the original parti- 

 tion by which the two halves of the splanchnocele cavities were separated from 

 one another. The cavities in the abdominal region come into communication 

 with one another by the veYy early disappearance of the partition on the ventral 

 side of the intestine. But it should be noted at once that a portion of this primi- 

 tive ventral partition, or, as we may call it, ventral mesentery, persists perma- 

 nently in relation to the position of the liver. 



8. The position of the liver. The primitive large veins of the embryo pass 

 through the septum transversum, and it is in connection with these veins, and 

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



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 abdominal cavity. 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 duct which is closely 

 parallel to the first, but which has no connection with any of the excretory appa- 

 ratus, and is destined to serve later as the female genital duct. In no inverte- 

 brate have we found anything homologous with these two ducts. 



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

 called auditory organs. 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 vertebrates. 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. 



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 ante- 



