THE ASCIDIANS AND AMPHIOXUS II5 



the egg before and just after fertilisation there is a considerable capacity 

 for regulation, which can be shown to involve mutual interactions be- 

 tween one part and another. 



Evidence has also accumulated that even after the ooplasms have be- 

 come localised, reactions between the parts of the embryo are by no means 

 at an end. The details of Dalcq's experiments with fragments of unfertil- 

 ised eggs had already suggested this possibility to him, and conclusive 

 evidence of regulation in the 2-cell stage was found by von Ubisch (1938), 

 who was able to cause two eggs at this stage to fuse together, when in 

 some cases only a single perfectly normal larva was developed. 



Much fuller information has been obtained from experiments in which, 

 at the 8-cell stage, the blastomcres have been separated and recombined 

 in various ways (cf. Rose 1939, Reverberi 1948, Reverberi and Minganti 

 1953). Quite a complicated cross-fire of interactions has been discovered. 

 If the first eight cells are separated into couples, as in Fig. 7. 4a, develop- 

 ment is not fully mosaic, since no brain, adhesive organs or eye-spots 

 appear in the larva from the anterior animal couple, nor any spinal cord 

 from the anterior vegetative couple. The other experiments summarised 

 in that Figure show that the brain, eyes and adhesive organs are induced to 

 develop from the anterior animal cells by some influence proceeding from 

 the anterior vegetative couple, but this influence is not effective on posterior 

 animal cells. In the standard embryological terminology, we should say 

 that only the anterior animal cells are competent to react to the stimulus 

 from the anterior vegetative blastomeres. There is some evidence that the 

 interaction is still more complex, in that there may be an influence from 

 the posterior vegetative blastomeres tending to iiiliibit the development 

 of the brain in the cells immediately above, even if these are an anterior 

 couple. The formation of the spinal cord of the nervous system is also 

 dependent on reactions between different regions, but in this case it is a 

 derivative of the anterior vegetative blastomeres which does not develop 

 when isolated from the anterior animal cells. 



We therefore fmd that, far from the ascidian egg being a complete 

 mosaic, it first undergoes a period in which a number of ooplasms become 

 locaHsed, and then enters one in which, although several of the regions 

 are already endowed with a capacity for independent differentiation, 

 some others are still labile, and develop normally only if certain reactions 

 take place. This lability affects particularly the nervous system, which 

 has two rather distinct parts, the brain and spinal cord, while the eye- 

 spots and adhesive organs also become fixed in their developmental fate 

 at the same time. We shall see that in vertebrate embryos the development 

 of the nervous system, and of many other organs which depend on it, is 



