July 1, 1889.] 



KNOWLEDGE ♦ 



185 



by the large cells, called the hypoblast, while the small 

 cells, known as the epiblast, form a kind of primitive 

 skin round the animal, and possess each little filaments 

 called cilia, serving for locomotion. These two layers pass 



A. Protozoon-Ophrydium. 



1st stage, a, nucleus, 

 section through body A 

 P, hypoblastic cells. 



Fig. 1. 



a, retracted. Al. Eakthworm, 



B. Metazoos — Hydea, transverse 



•B. a, ectodermic or epiblastic cells ; 



Bl. 2nd stage, transverse section. 



epiblast cells ; 0, hypoblast ; y, raesoblast. C. Longitu- 

 dinal section, a, epiblast ; $, hypoblast ; y, mouth ; 5, tentacle. 

 C 1. Vertical section, a, epiblast ; /3, hypoblast. 



into one another at the oval, or mouth-opening. The outer 

 layer is the organ of support of locomotion (by the cilia) 

 and may be the seat of the respiratory functions. Let me 

 here remind you that the frog — .so highly developed com- 

 pared with our earth-worm — under certain conditions 

 breathes largely by means of its skin. Also, in so far as 

 this outer cell-layer perceives the state of the surrounding 

 medium — that is, the air, water, or earth wherein it 

 dwells — it is an organ of sensation too. The inner layer is 

 nutritive in function ; it produces changes in the food 

 which is taken in, and allows what can bo turned to u.seful 

 account to pass into its cells, and these in their turn feed 

 the outer layer of cells. What is usele.-'s is passed out 

 again by the same opening whereby it entered. As the 

 functions of the two layers are different, so also the form of 

 the cells differs in each layer. And here wo may pause for a 

 moment to compare these stages with what we know of the 

 animals lower in the scale than earth-worms — i.e. the 

 Protozoa and the Coelenterata. One great law of develop- 

 ment is this : that each animal, in the commencement of its 

 separate life, passes through stages which are permanent 

 among animals lower than itself. There are two great 

 classes of animals : the Protozoa, which includes the Amreba, 

 the Infusoria, and the like ; and the Metazoa, wherein are 

 all other kinds of animals. One of the facts which mark 

 off the first class from the second is that a protozoon may 

 consist of any number of cells ; but those cells are not 

 different from one another either in form or function. It 



is very interesting to compare one of these, say ophrydium, 

 with the earth-worm's egg in its early stage. In both 

 you will find many cells cohering together, but in neither 

 will there be the least difference in the cells themselves or 

 in what they do. This proce.ss of cell-division in the egg 

 may therefore be explained as a survival transmitted from 

 early ancestors. Now the metazoa are characterLsed by 

 having at least two layers of differentiated cells in their em- 

 bryos. There is the ca-lenterata, which contains the common 

 hydra of ponds, the fresh-water polyp, the jelly-fishes, and 

 the sea-anemones. Let us take one of these, say a hydra, 

 and set it beside our young earthworm, thLs bag, with its 

 double layer of cells, for a coat, and we find that, as far as 

 form and functions go, they are almost the same. Our 

 hydra is nothing else than a stomach-bag with a double 

 coat, set on a stalk and furnished with tentacles. There 

 could hardly be a better illustration of the law of develop- 

 ment I have just mentioned. Later on, a third layer of cells 

 may be seen, the mesoblast, between the other two. 



It is a universal rule that from the outer layer of cells 

 (the epiblast) are derived the whole of the nervous portions 

 of an animal's body, as well as the .skin and the lining 

 membrane of the mouth, while from this mesoblast, or 

 middle layer, come the muscles, bones, connective tissue, 

 and the like. From the hypoblast is derived the epithelium 

 of the alimentary canal and of all the glands which are in 

 connection with it, such as the spleen, liver, and pancreas. 



Very soon the mesoblast of our young earth-worm on 

 one side of the bag-like structure becomes divided into a 

 number of square masses, disposed quite regularly on each 

 side of the middle line on the under surface of the 

 animal. Also, along this line, the outer layer of cells 



a, nerve cord; /S, epiblast; S, mesoblast ; y. hypoblast. 



thickens upwards and inwards, and from this is formed the 

 nerve-cord which runs the whole length of the body. From 

 it is derived the brain and s^iinal cord of a mammal. At 

 the same time, each of these square masses becomes hollow 

 in the centre, and somewhat like a sac itself. The adjoining 

 walla of each of these sacs fuse together, and here you have 

 a trace of the division into segments, which is common in 

 the animal kingdom, and whiih is shown in man by the 

 transverse bands across the abdomen. These sjic walls are 

 represented outside on the body of the earth-worm by the 

 transverse rings. On the under surface of the first segment 

 is the mouth ; we distinguish one surfice from tiie other by 

 the lighter colour of the lower one, and because on it in 

 each segment are four pairs of hairs or setie. These are 

 of great importance to the animal ; by their aid it can get 

 over the ground at a tolerably rapid rate, and when once 

 partly in its burrow, it holds so fast that it cannot be pidlod 

 out without being torn in pieces. 1 mentioned the gland — 

 the clitellus — it shows as a thickening of the skin between 

 the twentieth and thirtieth segments. This secretes a kind 

 of viscid gumliko fluid, which hardens and envelopes the 

 egg like the case of a cocoon. Let us now look at the digestive 

 system by the aid of the following diagram. At the upper 

 end you find the mouth, with a sort of lip to it, which is 

 useful for catching hold of food. Next comes a strong 



