THE VERTEBRATE TYPE OF STRUCTURE. 5 



by the nervous system. For instance, in insects or in the jointed worms (annelids) 

 there is a brain, so called, above the mouth, and a strand of nervous tissue 

 running down on either side of the body past the mouth to join the ganglion 

 on the lower side, thus completing a circumoral ring of nervous material through 

 which the oesophagus passes. In vertebrates, on the other hand, the mouth is 

 not enclosed by any cesophageal ring, and the entire nervous system is on one 

 side of the body and dorsal to the mouth. 



6. The division of the primitive body-cavity. The body-cavity in the em- 

 bryo is known by the comprehensive name of the ccelom. It will not be possible 

 to acquire a clear idea of its division until the embryos are actually studied. It 

 forms many parts. Of these there are two dorsal series, one on each side of the 

 central nervous system, which form cavities of what we designate as the somites of 

 the body. There are also two large ventral divisions which extend from the 

 region of the head to that of the future pelvis, one division for each side of the 

 body. These two large parts are not divided into segments at all, though the 

 cavities of all of the segments are primitively connected with these two main 

 divisions. Comparatively early in^J;he development the two main cavities be- 

 come connected with one another,\so as to constitute a single cavity to which we 

 apply the name of splanchnocele. The splanchnocele surrounds the heart of 

 the embryo, where we recognize it as the pericardial cavity, and it extends through 

 the future abdominal region, where we recognize it as the abdominal cavity. 

 The pericardial and abdominal regions of the cavity are separated from one 

 another in the embryo by a broad transverse partition which bears the name 

 of septum transversum. This septum in mammals becomes in the adult the dia- 

 phragm. It is one of the most striking of all the morphological peculiarities 

 by which vertebrates are distinguished from invertebrates. 



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

 tract of vertebrates into two fundamental parts, stomach and intestine, 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 elongated oeso- 

 phagus occurs in the higher vertebrates only, and is not a general characteristic 

 of the subkingd<3m. The mesentery by which the intestine is suspended to the 

 dorsal wall of the abdomen is the survival of the original partition by which the 

 two halves of the splanchnocele were separated from one another. The cavities 

 in the abdominal region come into communication with one another by the very 

 early disappearance of the partition on the ventral side of the intestine. But it 

 should be noted at once that a portion of this primitive ventral partition, or, as 

 we may call it, ventral mesentery, persists permanently 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 by intercrescence with these veins, and 



