130 HYPOTHESIS OF THE DEVELOPMENT OF 



with those attending the production of the individual organism. 

 This leads us to consider facts attending the embryonic develop- 

 ment of animals. 



First surmised by the illustrious Harvey, afterwards illus- 

 trated by Hunter in his wonderful collection at the Royal 

 College of Surgeons, Embryonic Development has latterly 

 become a science in the hands of Tiedemann, St. Hilaire, and 

 Serres. For some years its primary propositions were these : 

 1. The embryos of all animals are not distinguishably different 

 from each other; 2. Those of all animals pass through a series 

 of phases of development, each of which is the type or analogue 

 of the permanent configuration of tribes inferior to it in the 

 scale. The latter proposition was asserted to be more parti- 

 cularly true of departments of the organization, as the nutritive 

 system, the vascular system, the nervous system, &c., each of 

 which is destined for a peculiar degree of development in dif- 

 ferent groups of animals, according to their several needs. It 

 is now seen that while the first proposition remains as origi- 

 nally stated, the second must be modified. The whole truth 

 may be thus set forth : 



All organisms, vegetable as well as animal, commence with 

 a simple cell, of which it is impossible to tell in any case to 

 what form it is destined to advance. A series of changes 

 takes place. First, of an animal embryo, we can distinguish 

 whether it is destined for the radiate, molluscous, articulate, or 

 vertebrate sub-kingdom. Take an embryo of the vertebrate 

 sub-kingdom, we next trace in it the change which will deter- 

 mine whether it is to belong to the fish, reptile, bird, or 

 mammal class. Take an embryo of the mammal class, the 

 characters of the particular order are next determined. After- 

 wards, those of family, genus, species, sex, and individual are 

 evolved in succession. Thus, in the words of Von Bar, who 

 first enunciated the doctrine, " A heterogeneous or special 

 structure arises out of one more homogeneous or general, and this 

 ~by a gradual change" Thus, also, it must be clearly seen, the 

 embryo of each grade of being passes through the general con- 

 ditions of the embryos of the grades beneath it. As an illus- 

 tration : " There is no essential difference between the vertebral 

 column of the early embryo of man, and that of an embryo 

 fish ; the evolution of the nervous centres begins in both upon 

 the same plan ; so also does that of the circulating apparatus. 

 . . . ." x Some illustrations of this principle, as far as appli- 



1 Carpenter's Gen. Phys. 348. 



