CHARACTERISTIC VERTEBRATE STAGES. 39 



been passing through the coelenterate and arthropod stages of its development, the 

 later ones, as we have represented them, being essentially like those of Limulus and 

 the scorpion, although not presenting any characters foreign to a vertebrate. In 

 the following stages, the vertebrate characters appear. 



The changes that most affect the general shape and appearance of the brain 

 are the transfer of the lateral eye placodes to the inside of the cerebral vesicle, 

 and the union of the optic ganglia over the neural surface of the brain to form the 

 ganglion habenulse and the tectum opticum, or optic lobes. (Fig. 28.) As the 

 latter increase in size, they crowd the stomodaeal commissure and its ganglion 

 backward, over the posterior part of the midbrain region, where they form the 

 rudiment of the cerebellum. (Figs. 29, 36, 107 and 108.) 



When the convex, kidney-shaped lateral eye placode is transferred to the 

 brain wall, it becomes a concave, horseshoe-shaped retina. Still later, it becomes 

 a circular one, with a median fissure and centrally located nerve, both conditions 

 being the direct result of its ancestral shape and mode of growth. (Fig. 106.) 



The Auditory Pit develops from a prominent, disc-like placode, or segmental 

 sense organ, which in Limulus lies on the cephalothoracic shield, opposite the 

 third or fourth thoracic segment. (Figs. 29-30.) With the concrescence of the 

 anterior oral arches on the hsemal side of the head, the disc shifts its position to 

 that part of the head where it makes its first appearance in vertebrate embryos. 



(Figs- 33-34-) 



The Heart has been formed in typical arthropod fashion, by the concrescence 

 of the lateral plates of the vagus, and the anterior abdominal metameres, which 

 accounts for the fact that, in both arthropods and vertebrates, the anterior heart 

 nerves arise from the corresponding vagus and branchial neuromeres. (Figs. 32 

 and S3.) 



In the formation of the vertebrate heart, new factors may arise in the greatly 

 increased volume of the yolk sphere. In this case the branchial metameres, lying 

 near the equator of the egg, must extend their lateral plates completely round the 

 yolk before a heart segment can be formed. Hence, in the larger yolked eggs, it 

 is only the vagal and anterior branchial metameres that are in a position to form 

 heart segments in time to nourish the growing head structures. If the younger 

 and shorter caudal metameres produced heart segments, they would necessarily 

 arise later and would be separated from the head by the barrier of the abdominal 

 yolk navel. (Fig. 34.) The heart would then be a single tube at either end and 

 a paired tube in the middle. (Figs. 23, 139, C.) 



The thoracic, and the post branchial cardiomeres have been eliminated in 

 vertebrates, as they have been in arachnids, and the heart develops, as nearly as 

 one may determine, from about the same group of vagus and branchial metameres 

 in both cases. But the posterior end of the vertebrate heart still extends partly 

 round the yolk navel; hence the divided posterior end and divergent vitelline 

 veins; and it is crowded into an area on the haemal surface that is growing shorter 

 while the heart is growing larger, hence the auriculo-ventricular curvature. 



