TYPES AND LAWS OF FORM. 115 



may occur. Lastly, the lowest, or Gregarinidous form consists of a simple 

 gymnoplast, or naked nucleated cell, composed of a minute mass of protoplas- 

 mic sarcode, containing a single nucleus, with its included nucleolus. 



The science of philosophical anatomy further endeavors to penetrate 

 the fundamental laws at work in these several plans of construction. 

 Considered generally, it is seen that all animal forms exhibit a more 

 or less axial mode of growth ; that 'all are more or less perfectly bilat- 

 erally symmetrical, even the spirally convoluted Gasteropodous Mol- 

 luscs presenting two, though unequal, halves, and the radiating Echi- 

 nodermata also exhibiting an imaginary median plane of partition. 

 It is further seen that the spheroidal form is common amongst simple 

 animals, ova, and commencing tissue-cells ; the radiate form, in many 

 of the lower animals; and jtrara. arrangements, in others, as well as in 

 the organs or parts of animals higher in the scale ; lastly, repetitions 

 of similar parts are found to prevail in the elongated animals, and in 

 many organs and tissues. 



Furthermore, it is shown, on comparison, that like parts in the same 

 animal, or in different animals, may be extremely modified in form and 

 structure, to suit different purposes, and yet not lose their essential 

 identity ; in such case, they are said to be homologous parts. Thus, 

 each vertebra, or vertebral segment, is homologous with every other 

 vertebra, however highly developed these may be, as in the back, or 

 however they may be simplified, as in the sacrum or coccyx : even the 

 cranial segments, specially modified as they are, have been regarded 

 by many as homologous with vertebrae. Again ; the upper limb of 

 man, and its several bones, are homologous with the lower limb and its 

 bones, part for part, although the one is fitted for prehension and the 

 other for locomotion. The shoulder-bone and collar-bone together are 

 the homologues of the hip-bone ; the humerus, of the femur ; and the 

 radius and the ulna, of the tibia and fibula. Again, the carpus and 

 metacarpus are homologous with the tarsus and metatarsus ; the three 

 phalanges of the four fingers with those of the four outer toes ; and, 

 lastly, the thumb, having two phalanges, is the homologous part to 

 the great toe, which also has two phalangeal bones. The same law of 

 fundamental homology is evident in comparing, not merely the parts 

 of the same animal, but those of the different animals of the same type 

 with each other ; thus the upper prehensile limb of man is homologous 

 with the equally prehensile arm of the ape, with the locomotive fore- 

 limb of the mammalian quadruped and reptilian lizard, with the wing 

 of the bat and the more specially modified wing of the bird, with the 

 anterior flipper of the seal, the single flipper of the porpoise, the pad- 

 dle of the turtle, the fore-foot of the newt, and the pectoral fin of the 

 fish ; for all these parts, however different in form and use, are modi- 

 fications of the same fundamental portion of the vertebrate frame, 

 i. e., of the anterior lateral appendage. So also the lungs of the mam- 

 malia, birds, reptiles, and amphibia, are homologous with the air-blad- 

 der of certain fishes, though this ceases in most cases to be in any way 

 a respiratory organ. Passing from the Vertebrate to some other type, 

 as e. g., to the Annulose, we find homology in the different somites of 



