674 EMBRYOLOGY I INVERTEBRATES 



embryo of one animal and the adult of a structurally simpler 

 type. For example, a typical gastrula, such as that of Branchio- 

 stonia, has a formal resemblance to Hydra, although it differs 

 much in detail — the endoderm cells are not digestive, there are 

 no tentacles, and so on ; the embryos of amniotes possess visceral 

 clefts, which are found in fishes not only in the embryos but 

 also in the adults. 



In all the higher Metazoa, known as the Triploblastica, the 

 embryo passes through a stage where there are three more or 

 less distinct sheets of cells, the hypoblast, mesoblast, and epiblast. 

 These give rise to groups of tissues called respectively endoderm, 

 mesoderm, and ectoderm, and there is a general constancy of 

 origin of the various organs from one or other of these layers. 

 There are, however, many exceptions (voluntary muscle, for 

 example, is generally mesodermal but occasionally ectodermal) 

 and the old-fashioned rigid germ-layer theory, which postulated 

 an absolute constancy of origin for each tissue from one layer, 

 can no longer be maintained. 



The descriptive embryology with which we have chiefly been 

 concerned in this chapter seeks only to give an accurate account 

 of the changes in form which take place between egg and adult, 

 but the more recent science of experimental embryology seeks 

 also to find out how these changes are brought about and con- 

 trolled. Although it has discovered many interesting facts, it 

 has so far scarcely begun to answer the fundamental question, 

 and the best that we can do is to suggest some of the influences 

 which act on the developing individual. 



In the first place, the species of the animal, and many of its 

 infraspecific characters, are determined by the genes in the chromo- 

 somes (Chap. 29), and although the maternal cytoplasm some- 

 times plays an important part, this in its turn is determined 

 largely by the chromosomes under the influence of which it was 

 formed. It is obvious enough that the genes can only act in an 

 appropriate physical and chemical environment — a hen's egg 

 will not develop unless it is kept warm, nor a frog's unless it is 

 in water — but the external control is much closer than this, 

 as small variations may cause alterations in the form of the 

 embryo ; for example, the addition of magnesium salts to the 

 sea water in which they normally develop causes fish embryos 

 to develop a single eye in place of their normal two. At much 

 later stages the environment may still be an important determin- 



