\j6 OUTLINE OF THE ARACHNID THEORY. 



Thus the embryo elongates, apparently, in two different ways; by true apical 

 growth at the original apex of the embryo, and by the concrescence of the adjacent 

 parts of the germ wall, behind the apex. Failure to recognize the meaning of 

 these two processes in vertebrates has led to much confusion. 



As the telopore is merely a locally exaggerated marginal growth, its products 

 are not primarily different from those of the germ walls that concresce behind it. 

 But it will be observed that in their derivation and in their serial arrangement, 

 the two sets of products stand in totally different relations to one another, and to 

 their surroundings. (Figs. 138, 157.) 



In vertebrates, as well as in arachnids, neither the telopore, nor the concres- 

 cing margins of the germ wall have anything to do with a true gastrula, nor is their 

 mode of growth comparable with the coelenterate method of gut formation. As 

 indicated elsewhere, post-anal concrescence is the inevitable result when a living 

 film, extending by apical and marginal growth, spreads over a spherical surface. 



The Nervous System follows in the paths first laid down by the expanding 

 germ layers. The procephalic lobes, developing from the territory of the primi- 

 tive cumulus, and the lateral nerve cords on either side of the line of apical growth. 

 A slipper-shaped medullary plate is thus formed that in both arthropod and verte- 

 brate embryos has essentially the same structure, location, and mode of growth. 

 In both types we may recognize the marginal sense organs of the procephalic 

 lobes and the central, stomodaeal infolding. This infolding, with a rostrum-like 

 elevation on its anterior margin, is frequently a conspicuous marking in the middle 

 of the cephalic lobes of amphibia (Rana, and Necturus) (Fig. 25.) 



In this stage, the olfactory lobes make their appearance as a deep fold across 

 the anterior border, and the edges of the neural crest begin to grow over the 

 lateral margins of the medullary plate. A little later, the thoracic appendages 

 appear as gill arches and external gills. (Figs. 26-28.) 



The Primary Sense Organs. — In the following stages (Figs. 27-34), the 

 primary sense organs on the margins of the cephalic lobes move into, or toward, 

 their final position, and the gustatory organs make their appearance on the inner 

 margins of the basal lobes of the thoracic appendages. The olfactory organs, 

 ol, 0, move toward the anterior median line but remain in the surface ectoderm, 

 outside the brain chamber; the two pairs of ocelH are caught in the palial 

 fold and carried onto the membranous roof of the brain vesicle to form the parietal 

 eye; the kidney-shaped lateral eyes He on the outer edge of the fold, not quite 

 inside of the brain chamber. (Fig. 27.) 



Meantime the cornua, c, of the thoracic shield and the edges of the abdominal 

 pleurites appear. (Figs. 28, 33.) 



The haemal side of the cephalothorax is unsegmented. The greatly thickened 

 germ wall in this region concentrates around a point between the anterior end of 

 the heart and the forebrain, where a great mass of cells, the remnants of the 

 haemal blastoderm, are engulfed in the yolk and absorbed. (Fig. 33, c.nav.) 



Vertebrate Stages. — Up to this point, our vertebrate-arachnid embryo has 



