THE vertebrates: the amphibia and birds 153 



the determination of the plane of bilateral symmetry is, in many ways, of 

 considerably greater importance than anything which happens in the 

 much more striking phenomena of cleavage. Amphibian eggs, having a 

 moderate charge of yolk, undergo cleavages which are definitely, but not 

 exaggeratedly, unequal. The first cleavage plane is vertical, and usually, 

 though not always, coincides with the plane of bilateral symmetry run- 

 ning through the middle of the grey crescent — a coincidence brought 

 about by the fact that the sperm has an influence, again considerable but 

 not quite always effective, on the plane of the first cleavage, just as it has 

 on the plane of the grey crescent. The second cleavage is also vertical, and 

 perpendicular to the first. In many species, the first two dorsal cells are 

 smaller than the two ventral ones, which is an indication that the cleavages 

 are based on a bilateral-symmetrical pattern, which seems to underlie all 

 vertebrate cleavages, although it is often difficult to distinguish. 



The third cleavage is horizontal, and the furrow lies above the equator, 

 so that the animal cells are smaller than the vegetative. This is the first 

 indication of the effect of the yolk gradient, whose influence is predomin- 

 ant throughout the remainder of the cleavages. These soon become irregu- 

 lar, and proceed faster in the animal than the vegetative region, so that the 

 difference in volume of the cells becomes progressively more marked. 

 At an early stage — about the fourth or fifth cleavage — a space appears 

 in the centre of the mass of cells, the so-called cleavage cavity or blasto- 

 coel. This, of course, lies above the equatorial plane, and, as the unequal 

 cleavages proceed, it not only increases in size, but shifts further and fur- 

 ther towards the animal pole. 



Cell division continues throughout the whole of embryonic develop- 

 ment, but the 'period of cleavage' is considered to end when something 

 else begins to happen. The first definite event which occurs to terminate 

 it is the appearance of the blastopore and the beginning of gastrulation. 

 By this time the egg, which at this stage is called the blastula, has become 

 a hollow ball, with a thin roof of animal cells covering a large cleavage 

 cavity or blastocoel, beneath which lies a floor of large yolky blastomeres. 



The most important processes which have been going on under cover 

 of the cleavages have been two. Firstly, the divisions have cut up the 

 egg into cells of a size more attuned to that of the nuclei; and there has 

 been a considerable increase in the total amount of nuclear material, and 

 perhaps a synthesis of DNA, to assist in. bringing nucleus and cytoplasm 

 back to their normal relations (but see p. 58). Secondly, there has been a 

 considerable movement of material ; if vitally stained marks are made on 

 the vegetative pole of the egg, the dye is gradually carried right into the 

 body of the egg, and eventually reaches the floor of the blastocoel. 



