Comparative Development. 35 



plus the modification. This plus is to be taken in the algebraic sense, 

 since every modification may not involve an increase in the complexity 

 of the organism or its functions, and the addition of a subtract! ve modi- 

 fication will produce retrogression, of which instances often occur. 



Embryology shows further that although there are animals of every 

 grade of organic and functional development, every one of them passes 

 through substantially the same modifications, as far as it goes, as the 

 rest do. This is the point already shown in the comparison of the em- 

 bryo germ forms with the mature animals of the protozoan and infuso- 

 rial types. Every mammal, including man, therefore, while passing 

 through the germ form rnorula, is in structure equivalent to a minute 

 amoeboid or rhizopod animal composed of homogeneous cells, of which 

 multitudes inhabit all waters. Next, man is a blastula germ, structur- 

 ally equal to the mature Norwegian flimmer-ball, and substantially equal 

 also to the blastula germs of many other animals belonging to different 

 sub-kingdoms, classes and orders, from animals as simple as sponges 

 and worms, to the more complicated amphibians and mammals. 

 Amongst some of the simpler animals, as corals, sponges, &c. , the em- 

 bryo, while it is a blastula germ, is of the nature of an independent 

 animal, and is locomotive and self supporting in this as well as in the 

 further stages of its development. 



When the human germ reaches the disc form, in which it consists 

 chiefly of the two primary germ layers, its structural value is that of a 

 large class of simple animals, the zoophyta or plant animals, hydrozoa, 

 corals, seafirs, jelly fish or medusae, &c. These are, at maturity, com- 

 posed of only the two skins, the entoderm and exoderm. In these ani- 

 mals the two skins are differentiated very slightly from each other, the 

 entoderm acting as the stomach and superintending the personal econ- 

 omy of the animal, its digestion, nourishment, &c. , and the ectoderm, 

 being on the outside, receives the impressions from the environment and 

 sustains the relations of the animal to the outside world. The differen- 

 tiation in many cases, is so slight, however, that the animal may be 

 turned inside out, and the two skins will readily exchange offices. 

 From this stage in the human being, the differentiation becomes pro- 

 nounced, the principle throughout the development of the embryo being 

 that indicated in the two skinned hydra, the outside skin developing all 

 the organs that relate to the environment and maintain communication 

 with it, as, the outside skin, the organs of locomotion, including the 

 bones and muscles which are necessary to it, the nervous system, brain, 

 and all the organs of sensation touch, taste, smell, sight and hearing: 

 while the inside skin maintains the internal economy of the organism 

 in the development from it of the machinery of digestion and assimila- 

 tion the stomach, liver, heart and other blood vessels. 



