92 PRINCIPLES OF EMBRYOLOGY 



Perlmann (1953) has indeed been able to detect, immunologically, the 

 appearance of at least one new antigenic substance (protein?) at that 

 time. 



The study of the roles of the various types of cytoplasmic particle has 

 only begun very recently and there is still much to do. Hultin emphasises 

 a point v^hich is probably of considerable general importance, namely 

 that we must think of the developing cell as containing populations of 

 mitochondria and microsomes which are continually reacting on and 

 influencing each other; and one may add, of course, that the nucleus 

 with its contained genes must be also involved in the same general net- 

 work of cause and effect. 



All the grafting experiments on the gradient system described above 

 were carried out on the early cleavage stages, before the embryo 

 has sixty-four cells. Isolations of animal and vegetative halves can be 

 made at later stages, and it is found that the divergence of the isolate from 

 its normal fate remains much the same until the early blastula stage 

 (about ten hours after fertilisation). From that time onwards, the develop- 

 mental fate of the regions rapidly becomes more fuced, so that, for instance, 

 an animal half isolated from a later stage develops only the normal sized 

 tuft of cilia, and a vegetative half only a normal gut. The developmental 

 fate is not entirely determined in such halves, since if micromeres are 

 implanted into animal halves isolated at various times, it is found that 

 they can induce some vegetativisation even in halves which, if left isola- 

 ted, would develop only into their original presumptive fate. The em- 

 bryonic materials are, however, rapidly losing their lability, and by 

 about sixteen hours after fertilisation, an animal half can no longer be 

 affected by implanted micromeres. It is interesting to note that if an 

 animal half is isolated at the 32-cell stage, and allowed to develop in isola- 

 tion for some time before the micromeres are grafted into it, it loses its 

 reactivity earlier than it would do as part of a whole egg ; presumably its 

 isolation from its vegetative partner allows its animal tendencies to become 

 fixed earlier. 



3. The dorso-ventral axis 



It was stated earlier (p. 48) that the determination of a dorso-ventral 

 axis, which confers a bilateral symmetry on the egg, is one of the funda- 

 mental steps in development. It may well be asked how, and at what 

 time, this step is taken in echinoderm development. As a matter of fact, 

 the evidence suggests that there is some trace of bilaterality even in the 

 unfertilised egg, presumably dependent on factors operating during the 

 maturation of the egg in the ovary. If such eggs, or early cleavage stages, 



