OUTLINE OF THE ARACHNID THEORY. 





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FIGS. 24 to 34. A hypothetical series of arachnid and vertebrate embryos. The purpose of the series is to 

 show the continuity in the methods of growth and organic differentiation in vertebrates and arachnids. It begins 

 with the typical arachnid stages and leads up to those characteristic of primitive vertebrates, where without inter- 

 ruption they are carried on to completion. 



FIG. 24. A shows the radially symmetrical germ disc, or primitive cumulus, with its centrally located gas- 

 trula ingrowth; B, beginning of apical, or teloblastic, growth, and the appearance of bilateral symmetry; C, the 

 formation of the medullary plate; the unequal expansion of the thickened margin of the germ disc, or germ wall, 

 g. w., and the infolding of the teloblast to form the telopore, /. p. 



FIG. 25. The open medullary plate stage, with its neural crests, the marginal infoldings that mark the beginning of 

 the forebrain vesicle, and the forebrain sense organs on the outer slope of the neural crest. 



FIG. 26. Shows the appearance of the thoracic appendages; the segmentation of the lateral plate mesoderm 

 in the abdomial region; the beginning of the postanal concrescence of the germ wall; and the infoldings for the 

 middle cord, or notochord. The telepore is replaced by a primitive streak. 



FIG. 27. Shows the appearance of the gustatory lobes, the vagus and abdominal appendages, and the eleva- 

 tion of the caudal lobe. The olfactory organs have moved forward, in front of the head, and the median eyes 

 have been transferred to the inner limb of the neural crest. The cerebellum appears as the suprastomodceal 

 commissure. 



FIG. 28. The palial fold has covered nearly the whole of the forebrain, and the optic ganglia are crowded 

 backward and upward toward the oral region. The pleural folds appear, and the thoracic folds, or the thoracic 

 shield, extend over the posterior thoracic appendages, forming the beginning of the opercular or branchial fold. 



FIG. 29. The optic ganglia have united over the stomodaeum to form the tectum opticum. The neural crests 

 of the branchial region have closed, except over the mouth which lies just behind the stomodsal commissure, or 

 cerebellum. The uncovered space marks the location of the choroid plexus of the fourth ventricle. 



