in the Animal Kingdom. 361 



Again, if A represent a point near the middle of each arc, 

 the curves shew the development of a process originating there, 

 now into an arm or leg, and now into an upper jaw.* 



The diagram (Fig. 11) may be applied also in a comparison 

 of various parts arising out of the mucus (Plate I. Figs. 3 and 

 4 ***, Fig. 5, /, Fig. 6 and 7,^), in close union with the vascu- 

 lar (Plate I. Figs. 4 and 5, h, i, k, Figs. 6 and 7, e) lamina of 

 the germinal membrane of Man. 



These united lamina? having become a tube, there occurs in 

 certain sections diminished, in certain others increased Fig. is. 

 growth, by which organs are originated, presenting 

 the appearance of processes, in the one case towards 

 (Fig. 15), in the other/row (Fig. Ifi) the axis of the Fi * 1& 

 tube ; these processes having, as their base, either c{ )=j 

 the whole or a part only, of its circumference. 



Some of these processes are in no small degree analogous to 

 each other, and, as is the case with all other organs of a series, 

 with all animals indeed, the more alike, the nearer to the period 

 of their origin we view them. 



Thus the lungs have been compared to the urinary organs, 

 and the genitals to portions of the alimentary canal.-f But ha- 

 ving proposed to enter upon general considerations only, we can- 

 not go into the details of comparison. 



With this common origin,' and therefore coincidence in fun- 

 damental and general form, it is not surprising that organs 

 should present analogies. Besides which, there seems, however, 

 unity of plan in proceeding from the general to the special, a 

 tendency, as said before, to repetition of parts in the several sec- 

 tions of the same tube. Development appears to take the same 



* Or in some of the Vertebrata, into a wing, a fin, &c. 



In Fig. 10, p. 131 of the former memoir, there was given an ideal transverse 

 section, shewing the structures formed out of the serous lamina of the germi- 

 nal membrane in the Arthrozoa. There is a remarkable tendency to repeti- 

 tion in the segments of their dermo-skeleton, including that portion of the 

 latter that enters into the formation of the head; and in certain of them the 

 legs insensibly pass into maxillae. No doubt the same tendency to repetition 

 is universal in the animal kingdom. 



t The resemblance is very striking in animals of simple structure, as well, 

 indeed, as in many of the Vertebrata ; in which, for example, the oviducts are 

 scarcely distinguishable from parts of the intestine. 



