290 STUDIES IN THE EVOLUTION OF ANIMALS 



All students of embryology know that the ovum, in developing, 

 divides into two cells, then into four, then into eight, then into six- 

 teen, and subsequently into a number of cells called a ' morula,' 

 and further on a ' gastrula,' after which the embryo begins to take 

 shape. 



By these experiments it has been shown that 'the germ is 

 plastic in the grip of its environment, and various malformations 

 have been induced which are of interest to the student of mor- 

 phology ' ; that ' there is now good evidence to prove that these 

 disturbing agents act, at least in the majority of cases, on that part 

 of the developing organisation which is concerned with the forma- 

 tion of the vascular system of the embryo ' ; ^ that ' of the first two 

 cells into which the egg of a Frog develops, one has in it the 

 material for forming the right half of the body, the other has in it 

 the material for forming the left half of the body.' It was proved 

 that ' one of the first two segmentation cells may form half an 

 embryo, that it can develop apart from its neighbour, and that 

 either a right or a left half-embryo might be produced, as well as 

 an anterior or posterior half ; that, quite lately, Roux has been able 

 to rear an entire Frog embryo from half an egg ' ; in experiments 

 on ova of an Ascidian, Chabry found that 'by destruction of one 

 of the cells into which the ovum first divides, the remaining cell 

 develops into a half-larva, and if two anterior cells of the four- 

 celled stage be destroyed, a posterior half-individual results ' ; that 

 ' on to the sixteen-cell stage at least, each cell has a determined 

 destiny, and represents a definite part of the embryo, and that if 

 one of these sixteen cells be destroyed, the defect (or monstrosity) 

 in the larva is a definite one ' ; that ' while Roux got half-embryos 

 from half an egg, Driesch got half-sized, but otherwise complete 



1 Surely the vascular system is under the control of the nei-vous system, or of some 

 electrical influence which corresponds to it, before the regular nervous system is 

 developed. 



