338 ANIMAL DEVELOPMENT 



spurious passages interpolated by a later hand. The embryo- 

 logical record is almost always abbreviated in accordance with the 

 tendency of nature (to be explained on the principle of the survival 

 of the fittest) to attain her ends by the easiest means. The time 

 and sequence of the development of parts is often modified, and 

 finally, secondary structural features make their appearance to fit 

 the embryo or larva for special conditions of existence. When the 

 life-history of a form is fully known, the most difficult part of his 

 task is still before the scientific embryologist. Like the scholar 

 with his manuscript, the embryologist has by a process of careful 

 and critical examination to determine where the gaps are present, 

 to detect the later insertions, and to place in order what has been 

 misplaced." 



The difficulties mentioned above at once beset us when we 

 attempt to explain on an evolutionary basis the processes through 

 which the egg-cell goes immediately after fertilization. These 

 consist of a series of divisions, technically known as cleavage or 

 segmentation (not to be confounded with segmentation of the adult 

 body), by which a little mass of cells, i.e. a blastula, is brought into 

 existence. Nothing could be easier than to assert that this stage 

 corresponds to the colonies of Protozoa which evolved into the 

 earliest Metazoa, and such an assertion would no doubt be true 

 in a general way. But it is rather disappointing to find that the 

 blastula differs greatly in animals of different kind. Sometimes it 

 is spherical and solid (morula), sometimes spherical and hollow 

 (blastosphere\ or it may be a two-layered plate of cells {plac^da). 

 And occasionally it is not composed of cells at all, but is a mass of 

 protoplasm containing numerous nuclei, which serve as centres 

 round which cells are later on marked out. To decide which of 

 these forms is primary, i.e. really represents an early ancestral 

 stage, and which are secondary, i.e. modifications due to the exi- 

 gencies of development, would puzzle Solomon himself, supposing 

 that worthy to be posted in the facts and methods of modern 

 science. And, as often happens, the triumph of one view over the 

 rest might conceivably be not so much a contribution to fact as a 

 tribute to the ability or ingenuity of some forceful authority. As 

 the late Professor Huxley was wont to remark, in his characteristic 

 way: " Theories which at first blush appear to be the most complete 

 and beautiful are commonly those which are most easily upset". 

 Without attempting to support the claims of any one kind of 



