HUMAN RESEMBLANCES TO LOWER LIFE. 19 



life with which this remote ancestral stock was favoured, occurred 

 when the structureless animal of the first period became the "gastrula- 

 cup " of the second. It is in this fashion that we are led to see in 

 the development of man or of any other animal form, a panorama of 

 the evolution of its race. We can thus also conceive how meaning- 

 less on any other supposition in the eyes of naturalists, are all those 

 interesting stages which herald the genesis and production of each 

 individual animal or plant that is born into the world. 



An interesting episode in the history of human genesis and of 

 vertebrate development at large, consists in the further observation 

 that at an early period after the " gastrula stage," and, indeed, even 

 contemporaneously with the appearance of that stage itself, the cells 

 of the primitive body arrange themselves in two layers, an outer and 

 inner, whilst a third layer is in due time formed between them. 

 From these three layers, and by their subsequent elaboration to 

 form the fundamental body-substance, all the organs and parts of 

 animals are formed. The outer layer is named the epiblast. From 

 this tissue are formed the outer or " scarf-skin," and the brain and 

 spinal cord. It is exceedingly curious to note that the most super- 

 ficial layer of our bodies, and the deeper nervous centres, are formed 

 from one and the same layer. The inner layer is known as the 

 hypoblast ; and from it are elaborated the lining membrane of the 

 digestive canal and that of the lungs. The middle layer receives 

 the name of the mesoblast. This latter structure may be credited 

 with forming the great bulk of the body. To it the bones, muscles, 

 bloodvessels, and viscera generally, owe their origin. Tracing back 

 our progress in the matter of individual formation, we thus arrive at 

 the conclusions that, to begin with, our bodies, however complex 

 they may appear, sprang each from a single cell, that this cell begets 

 many others, and that these cells, finally arranging themselves in 

 three layers, form the entire frame. And these facts, it may be 

 noted, apply with equal force to the genesis of a worm or a snail, and 

 to the fashioning of the human frame. 



Later on, the progress of human development is found to exhibit 

 certain features which, equally with the foregoing stages, testify to 

 the far back ancestry to which humanity owes its being. A survey 

 of one or two features of special interest in the development of lower 

 Vertebrates will preface in a natural manner the brief study of the 

 peculiarities of lower life in which man is found to share. When the 

 development of the fish is studied, we observe that at an early stage in 

 his history certain clefts appear in the sides of the neck, these clefts 

 being separated from each other by solid bars or partitions. The 

 clefts are known as the " branchial clefts " or " gill clefts," and the 

 bars as the " branchial" or "gill arches." From four to six pairs of 

 these clefts exist in the fish, and as fishes breathe by means of gills 



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