CEPHALOGENESIS. 5 



in the serial location and subsequent migrations of the primary cephalic sense 

 organs (median and lateral eyes, olfactory, and auditory organs); d. in the 

 degree of development of the cephalic mesoblast, and in the direction and extent 

 of its growth in the several regions; e. in the development of the heart;/, in the 

 concrescence of the so-called "lips of the blastopore," and in the growth of the 

 margins of the embryonic area (" germ wall ") ; g. in the formation of the head fold. 

 C. Arachnid Cephalogenesis Prophetic of the Vertebrate Head. We 

 shall show that the continuation, or the exaggeration, of the processes already 

 initiated in the arachnids inevitably leads to the establishment of the conditions 

 now seen in the vertebrates. For example: a. The further withdrawal of the 



noto. 



C 



per c. 



card.g. 



FIG. 2. Semi-schematic cross-sections of a marine arachnid, showing location of principal organs. .4, abdominal 

 region; B, branchial region; C, mesocephalic, or thoracic region. 



principal alimentary and urogenital organs of the arachnids into the postcephalic 

 regions, would produce the condition in vertebrates, b. The continued enlarge- 

 ment and closer union of the thoracic neuromeres, and their more precocious 

 development during embryonic periods, aided possibly by the further overgrowth 

 of the labrum and optic ganglia, would lead to a further narrowing, and over- 

 growth of the passage for the esophagus, and ultimately to the permanent closure 

 of the old mouth, as in vertebrates, c. The continued increase in the size of the 

 yolk sphere, the absence of mesodermic structures on the haemal side of the tho- 



