4:2 Dynamic Theory. 



Beginning with a mere nervous filament in the lowest organisms, it 

 appears in the star fishes with additional threads connected together so 

 that sensation is convej^ed from one part of the body to another. 



In the mollusks are found knots of nervous cells by which the sensa- 

 tions are connected with each other. In the worms, a large knot or 

 ganglion, placed above the throat and termed the throat ganglion, is 

 the equivalent of a brain. In the amphioxus there is a spinal cord 

 reaching from end to end. 



FIG. 63. Amphioxus lanceolatus. It has a pigment spot for an eye 

 over the front end of spinal cord also a small olfactory pit, no ear. C. 

 Cirri surrounding- the mouth. KS. Gills. Ov. Ovary. L. Liver. N. 

 Longitudinal folds supposed to act as kidneys. P. Pore of the gill 

 cavity. A. Anus. Ch. Notochord. RM. Spinal cord. 



In the cyclostomi ( lamprey, &c. ) there are five vesi- 

 cles in a row, constituting a brain. In the development 

 of vertebrates there are, at an early period, three brain 

 vesicles, which, as mentioned above, soon form five. 



From the three principal parts shown in figs. 64 and 

 65, all the rest are subsequently formed, the front form- 

 ing the fore-brain and twixt-brain, which finally become 

 the cerebrum and optic thalamus with their appendages; 

 the hind part forming the hind-brain and after-brain, 

 which, later, become the cerebellum and medulla oblon- 

 gata, while the mid-brain bladder develops the corpora 

 quadrigemma, the organs of sight. (See fig. 25.) Dif- 

 ferent parts of this complicated system of ganglions are 

 developed in greatly different degrees in different races, 

 according to their surroundings and habits of life. 



The development of the brain in the human embryo, 

 like the development of the other organs, passes succes- 

 sively through the various stages that are permanent in 

 the various types of vertebrates below, and at last the 

 brain is essentially that of the higher apes, possessing 

 no organ or part not possessed by the apes. Not that 

 there is no functional difference between the brains of 

 men and those of apes, but it may be asserted that the 

 difference is one of degree exclusively. 



But there is not only a very close correspondence of or- 

 gans and functions in the individuals of all the multifa- 

 rious tribes of vertebrates, and a still closer one between 

 those of the mammal tribes, but the same accessories of 

 tools, instruments, appliances and temporary scaffolding, 

 as it were, are used to build these organisms jn all cases. 

 It has been seen how all start from the impregnated egg, 

 and increase their size by segmentation and the absorption of nourish 



