5^8 EMBRYOLOGY : INVERTEBRATES 



and kidney are subsequently formed. The processes which take 

 place seem to be a movement of cells similar to that of gastrula- 

 tion and possibly caused by surface forces, adhesion of Hke 

 cells, and induction. The action of the organiser in initiating 

 gastrulation can be imitated by a wide variety of tissues, both 



living and dead, by many 

 chemical substances, and even 

 by extreme temperatures. The 

 simplest seems to be a salt 

 solution which induces a mild 

 cytolysis, and it is probable 

 that many of the more com- 

 plicated substances act in the 

 same way. The action of the 

 cytolysis may be to liberate 

 within the reacting cell sub- 

 stances, previously combined 

 and inactive, which then deter- 

 mine the fate of the cells as 

 nerve cells, ectoderm cells, or 

 whatever it may be. According 

 to this view the labile tissues 

 must contain substances which 

 can cause them to develop in 

 various ways ; it is an accident 

 of time and place which is 

 liberated and so has its effect. 

 The substances concerned, 

 though present in the cyto- 

 plasm, are perhaps akin to 

 genes. 



The later development of 

 tissues is, so far as we know, controlled in two ways. The first, 

 which has some resemblance, which may only be superficial, to 

 the control by organisers, is by the action of specific chemical 

 substances or hormones which circulate in the blood or diffuse 

 through the tissues to affect cells other than those which produce 

 them. Examples of hormones have been described in earlier 

 chapters ; thyroxine and the growth factor of the anterior pituitary 

 affect growth in size and proportion, and also have specific effects 

 on amphibian metamorphosis. Thyroxine administered to young 



r.set 

 a.v 



Fig. 531. — Section through an organ- 

 iser-graft in the newt Triton. The 

 anterior end of the secondary 

 embryo is at right angles to the long 

 axis of the primary embryo. — From 

 Huxley and De Beer, after Spemann. 



f.c, gut cavity ; l.sec, a.v., left ear vesicle of 

 secondary embryo ; pr. br., brain of primary 

 embryo; r.sec. a.v., right ear vesicle of second- 

 ary embryo ; sec. br., brain of secondary 

 embryo. 



