114 PRINCIPLES OF EMBRYOLOGY 



body-cavity originating in this way has been called an entero-coel, in 

 distinction to the schizo-coel (formed by spHttrng of an originally solid 

 layer) which is characteristic of most vertebrates. But it seems probable 

 that the distinction is not such a fundamental one as some of the older 

 embryologists thought. 



The eggs of ascidians have been favourite material for experimental 

 study for many years ; Amphioxus material is less easily come by and much 

 less is known about it, though what data do exist suggest that its epigene- 

 tic physiology is not greatly different from that of the better-known 

 group. The older workers found much evidence that the ascidian egg is a 

 typical mosaic type. In particular, Conldin (193 1) showed that after 

 fairly strong centrifugation which moved around the interior cytoplasm 

 abnormal larvae develop, their build corresponding to the new positions 

 to which the ooplasms had been shifted. He found, however, that the 

 essential factors which determine the mode of development of the regions 

 are not the inclusions to which the colours of the various crescents are due. 

 For instance, the yellow granules can be moved out of the yellow crescent 

 by mild centrifugation, but the mesoderm stiU appears in its normal 

 place so long as the hyaline ground-substance has remained in place. Ries 

 (1939, see also Reverberi and Pitotti 1939) later found that the property 

 of developing into mesoderm (and particularly muscles) is bound up with 

 some cytoplasmic constituent which has a high peroxydase activity; 

 possibly these are ultra-microscopical granules. Wherever the peroxydase- 

 containing material is forced to by the centrifugal force, there the muscles 

 will eventually develop. 



In recent years, it has become clear that the location of specific ooplasms 

 in definite regions of the egg is only one part of the story (Review : 

 Reverberi 1948). The first important additional information concerned 

 the mechanism by which the localisation is brought about. Dalcq (1932) 

 found that, if eggs are cut into two parts before being fertilised, two com- 

 plete larvae can be produced (provided the cut is meridional, so that each 

 fragment contains the whole animal- vegetative axis). This is a typical 

 'regulation', similar to that found in the echinoderms. It certainly indi- 

 cates that considerable redistribution of the ooplasms can occur in the un- 

 fertilised egg. Reverberi (1936) in similar experiments studied the capacity 

 of egg fragments to regulate sufficiently to carry out a normal bilaterally 

 symmetrical cleavage. He found that they retained this degree of flexi- 

 bility up to the time of the emission of the polar bodies, but that the 

 regulation is dependent on some factor located in the vegetative region, 

 in the absence of which the animal part alone cleaves as though it remained 

 only part of an egg, and does not develop bilateral symmetry. Thus in 



