90 ON THE MORPHOLOGICAL CONSTITUTION OF 



the " permanent vertebral system." Thus, the products of tlie 

 development of a single primordial vertebra are — 1. A pair 

 of spinal nerves, with their ganglions ; 2. The vertebral arch 

 and pair of ribs irmnediately behind this pair of nerves ; 3. 

 The anterior part of the body of the vertebra to which this 

 arch and ribs are attached ; 4. The intervertebral disk in 

 front of it ; 5. The posterior part of the body of the vertebra 

 in front ; and, 6. The group of spinal muscles between these 

 two vertebrae. The bones, muscles, and nerves of the abdo- 

 minal and thoracic wall are formed by an extension down- 

 wards, and adhesion of the lower or costal portion of the 

 "primordial vertebral system" to the inner surface of the 

 external of the two layers into which the " primary abdominal 

 wall" divides. This outer or adherent layer of the " primary 

 abdominal wall" becomes the areolar layer of the integument, 

 and enters into the formation of the limbs. The inner layer, 

 separated from the outer by the pleuro-peritoneal space, forms, 

 with its fellow of the opposite side, the Wolffian bodies, re- 

 productive glands, spleen, permanent aorta, mesentery, and 

 the muscular and serous covering of the alimentary tube. 



From these remarkable observations of Eemak, it would 

 appear that the sclerome of the hsemapod, from the anterior 

 part of the neck backwards, originates as a series of in- 

 dependent sclerotomes, and that, contemporaneously with 

 each sclerotome, a corresponding myotome and neurotome 

 take their rise in a common primordial segment of blastema. 



The cephalic portion of the early vertebrate embiyo is 

 peculiar, more particularly, according to Eemak, in the non- 

 appearance of distinct " primordial vertebrae," and of the 

 subsequent changes which result from their development. 

 The great divisions of the brain and of the cerebral nerves 

 indicate, indeed, the segmented character of the entire struc- 

 ture, but I am inclined to believe that, in the present state of 

 the subject, these indications are not to be depended on for 



