222 



CLASSIFICATION OF VERTEBRATES. 



tion being ovary, the posterior testis. The sexual products are 

 discharged into the body cavity, from which they escape to the 

 exterior through genital pores which open into the hinder end 

 of the urinary (pronephic) ducts. 



The eggs of the myxinoids are large, and each bears at either 

 end a crown of long-stalked anchoring-hooks. Almost nothing 

 is known of the development. The lampreys 

 have much smaller eggs, the early development 

 of which shows striking similarities to the con- 

 ditions found in the amphibia. The eggs un- 

 dergo a total but unequal segmentation, with 

 small cells (micromeres) at one pole, and larger 

 yolk-laden cells (macromeres) at the other. 

 Gastrulation is effected in a modified manner 

 by a growth of the micromeres over the mac- 

 romeres, and the blastopore (or rather its pos- 

 terior end) persists as the anus of the adult. 

 In the development of the nervous system, in- 

 stead of the typical inrolling of a medullary 

 plate there is formed a solid cord or keel of cells along the 

 middle line of the back, in which later, by splitting, a cavity 

 appears. At the extremity of the head appears an inpushing, 

 from the walls of which both olfactory organ and hypophysis 

 are developed ; and it is stated that the olfactory structures are 

 paired at first, the azygos condition of the adult being a secon- 



FIG. 226. 



Egg of Bdello- 

 stoina; at a , a 

 single hook 

 enlarged. 



FIG. 227. Palaospondylus (enlarged), from Dean, after Traquair. 



dary feature. The young hatches from the egg in a larval con- 

 dition known as the ammoccetes stage, with rudimentary eyes 

 and a large hood-shaped upper lip. This is later metamor- 

 phosed into the adult. 



Fossil marsipobranchs are imperfectly known. Formerly 



