1 8 STUDIES IN LIFE AND SENSE. 



and being in this guise practically undistinguishable from the germ 

 which is to develop into that of any other vertebrate form. What- 

 ever this germ or " ovum " may become, it presents in substance, 

 chemical composition, and microscopic characters, no features which 

 are. other than those seen with equal distinctness in that of every 

 fish, frog, reptile, bird, or quadruped. Humanity thus, as a matter 

 of bare, unadorned fact, starts from a structural platform which is 

 common to all the members of the great " backboned " group. The 

 primitive changes which occur in the history of this germ are next 

 noted to run in strictly parallel lines with those viewed in the develop- 

 ment not merely of all other vertebrates, but it may be added of all 

 other animals from the Sponges to the " backboned " series. The 

 changes in question are collectively termed the " segmentation " of the 

 germ. By this term is meant the division of the protoplasm of the 

 " germ " which in itself is one " cell " into a large number of 

 similar and smaller cells. This result has been very aptly compared 

 to the process of fashioning many similar bricks from the common 

 clay of a field. The primitive "germ" is such a field in miniature, 

 and the cells which result from its division are the units, through 

 the subsequent arrangement of which, the animal house in general, 

 and the human edifice in particular, are respectively built. There is 

 no reason to doubt that what has been seen to occur in the course 

 of the frog's development, and what is known to occur in the fashion- 

 ing of the developing rabbit a member of man's own class likewise 

 occurs in the outlining of the human form. As the course of develop- 

 ment therefore proceeds, we discover that the cells arising from the 

 division of the primitive germ arrange themselves to form a rounded, 

 and in lower forms cup-shaped body, whose wall is double, and 

 which has been named the gastrula. Here ensues an important 

 observation in so far as the genesis of animal forms is concerned. 

 It is known that in the developmental history of every class of 

 animals, from the Sponges to the " backboned " series, this cup-stage 

 or " gastrula " is represented. Hence, biological opinion, regarding 

 this universally occurring landmark in animal history, assigns to it 

 a high rank in the list of the guides which point out the paths of our 

 development and evolution in the past. We know of certain 

 animals among the Sponges and Zoophytes, which, never advancing 

 beyond this cup-stage, appear before us as permanent "gastrulas." 

 That which is a transitory stage in our own early history becomes, 

 when arrested, a permanent and adult stage of lower life. If, there- 

 fore, the story told us by our development is to be construed at all, 

 it must be held as showing that " once upon a time " our most 

 primitive ancestry began under the guise of a protoplasm -speck 

 represented to-day by the germ which forms the initial stage in our 

 history. Whilst the recital also clearly shows us that the first rise in 



