THE INTEENAL COAT THE NERVOUS TUNIC 



663 



discs by maceration as the expression of its mode of construction from 

 mitochondria. It remains uncertain whether the longitudinal fibrillation 

 of the internal segments is likewise the probable result of an arrangement 

 of mitochondria in the developing cell ; the complete history of these mito- 

 chondria has not yet been traced. 



The Inversion of the Retina. The receptive cells of the vertebrate 

 retina exhibit, in contrast to all other neuro-epithelial cells, a reversed 

 polarity with respect to the source of their special stimulus, the ether 

 waves; the transmitting end of the cells is nearer the source of the light 

 than the percipient end. The human retina is appropriately described as 



Visual cell 



Retinal layer 

 Pigment layer 



Infuniibulum 



Neural ectoderm 

 Neural canal (forebrairi) 

 Epidermal ectoderm 



t Optic cup 

 Optic stalk 



Rathke's pouch 

 Buccal cavity 



FIG. 562. DIAGRAM ILLUSTRATING BALFOUR'S THEORY TO ACCOUNT FOR THE 

 INVERSION OF THE VISUAL CELLS OF THE VERTEBRATE RETINA. 



Transverse section through the head of a hypothetical vertebrate embryo, to 

 show the morphological relations of the surfaces of the ectoderm of the integument, 

 the neural tube, and the forming retina. In each of these situations a single sense 

 cell is indicated. (After Parker, Amer. Nat., 42, 501, 1908.) 



an inverted sense-organ. The most plausible theory yet proposed in ex- 

 planation of this inversion of the visual cells seems to be the one outlined 

 by Balfour (1881), expressed in terms of the ancestral history of the verte- 

 brate eye. (Among the invertebrates an inverted retina is known only in 

 certain mollusca, e.g., Pecten, and in certain spiders and the scorpion.) 

 'According to this view the vertebrate retina originated on the outer sur- 

 face of the ancestral vertebrate in much the way that the eyes of many 

 invertebrates have been produced. The primitive retinas thus formed 

 were implanted in that portion of the surface of the animal from which 

 the central nervous system was destined to develop, and when this was 

 infolded these retinas were carried in with it and came thus to be involved 

 in the central organ. If the morphological position of a sensory cell, such 

 as may have existed in the primitive external retina, is supposed to have 

 been thus retained as this organ was carried from its superficial location 

 into the central nervous system and out again almost to the external sur- 



