46 



THE ORIGIN OF VERTEBRATES 



I 



by the growth of the nerve-masses on each side of it. Throughout 

 the whole vertebrate kingdom this obliterated portion still leaves 



its mark as the raphe or seam, 

 which is so characteristic of 

 the infra-infundibular portion 

 of the brain. 



Here, again, it is seen how 

 simple is the explanation of a 

 peculiarity which has always 

 puzzled anatomists — why 

 should there be this seam in 

 the infra-infundibular portion 

 of the brain and not in the 

 supra-infundibular or in the 

 spinal cord ? The correspond- 

 ing compression in the upper 

 brain-region forms the lateral 

 ventricles, as is seen in the 

 accompanying figure of the 

 brain of Ammoccetes (Fig. 22). 

 In yet another instance it is 

 seen how markedly the nervous 

 masses are arranged in the 

 same position with respect to 

 the central tube as are the 

 nerve ganglia with respect to 



Fig. 22. — Horizontal Section through 

 the Brain of Ammoccetes. 



Cr., membranous cranium ; I, olfactory 

 nerves; l.v., lateral ventricles; gl., glan- 

 dular tissue which fills up the cranial 

 cavity. 



the intestinal tube in the case 

 of the invertebrate. Thus in birds a portion of the spinal cord 

 in the lumbo-sacral region presents a very different appearance 



from the rest of the cord ; it is 

 known as the rhomboidal sinus, 

 and a section of the cord of an 

 adult pigeon across this region is 

 given in Fig. 23. As is seen, the 

 nervous portions are entirely con- 

 Pig. 23.— Section through Rhomboidal fi ne d to two masses connected 



together by the white anterior 

 commissures which are situated laterally and ventrally to a 

 median gelatinous mass ; the small central canal is visible and 



