ORGANIZERS AND TISSUE DIFFERENTIATION 271 



direction. The more anteriorly a tissue is situated, the greater is the variety 

 of tissues which it is able to produce ; in the posterior direction the frequency 

 and completeness in the production of such a variety of tissue are, step by 

 step, decreased. We have evidently to deal with a multiplicity of factors 

 which determine the formation of these structures and which also bring about 

 in the course of embryonal development a gradually diminishing receptiveness 

 of the tissues to the stimuli of the organizers. 



In some respects we observe here, in principle, the same conditions which 

 we found in the cervix of the guinea pig, where there is a gradual decrease 

 in the potency of the tissues in one direction to form uterine structures, and 

 in the other direction to form vaginal structures under the influence of hor- 

 mones. We may consider uterus and vagina as representing two opposite 

 poles. In passing from one pole to the other, or in the opposite direction, there 

 is a graded change in structure and in mode of reaction to hormones. 



Thus it is seen that there is a close correspondence between the action of 

 organizers and that of well known hormones, which occur in invertebrates 

 as well as in vertebrates, but which have best been studied in mammals. The 

 organizers represent hormones which are present and act locally in contact 

 with the recipient tissue, in contradistinction to distance hormones, which act 

 after being carried to a distant recipient organ; the former are contact hor- 

 mones produced in the cells and causing cytoplasmic differentiations in certain 

 responsive tissues with which they are in contact. These organizers are devoid 

 of the finer organismal differentials and there are indications that they may 

 not possess any organismal differentials. 



Further instances of correspondence in the action of contact and distance 

 substances may be cited : In the case of the corpus luteum it has been shown, 

 in the guinea pig, that a very interesting correlation exists between the time 

 during which the hormone produces the maximum effect on the recipient 

 tissue, namely, the uterine mucosa, and the period during which such a hor- 

 mone effect is needed for the embedding of the fertilized ovum. It is only at 

 a time when the hormone is produced in full strength that the tissue exhibits 

 its full responsiveness. After the period has passed during which the egg nor- 

 mally attaches itself, the recipient tissue loses its responsiveness to less specific 

 stimuli to which it was formerly responsive, presumably because the quantity 

 of hormone necessary for sensitization of the tissue is diminished, or because 

 a refractory state develops in the uterine mucosa. 



A somewhat similar condition exists in the relation between organizer and 

 recipient embryonal tissue. Here, as Lehmann has pointed out, the time during 

 which the organizer is produced in maximal quantity in the upper lip of the 

 gastrula of a certain species corresponds to the time when the ectoderm of the 

 gastrula, which is the recipient embryonal tissue, is responsive to the action 

 of the organizer. This correspondence applies, however, only if organizer 

 and recipient tissue belong to the same species ; it does not apply if organizer 

 and recipient tissue are derived from distantly related species ; in the latter 

 case, abnormalities may result. 



There are, however, other cases in which a hormone is still produced at a 



