EXPERIMENTAL EMBRYOLOGY 



79 



and the frog, where the first three cleavage-planes are at 

 right angles to each other, the first two being meridians 

 and the third equatorial. In other cases the blastomeres 

 may be unequal in size, so that a spiral type of cleavage 

 results, and many intermediate forms are known. In 

 insects and crustaceans there may be many nuclear divisions 

 without division of the cytoplasm, though ultimately the 

 nuclei separate and each becomes the centre of a cell. 



The two first blastomeres frequently correspond to the 

 right and left halves of the future embryo, but exceptions 

 to this are very common, and there may be much variation 

 in this respect even within a single species. Conse- 

 quently the problem of the origin of the bilateral sym- 

 metry of the embryo is not the same as the problem of 

 the determination of the first cleavage plane. The eggs 

 of squids and of many insects have a bilateral shape and 

 structure before fertilisation, and the axes of symmetry 

 of the egg are the same as those of the embryo : cases 

 have been described in which insect eggs are laid in lines, 

 all pointing in the same direction, and the development 

 can be followed through the transparent shell until there 

 is an Indian file of unhatched larvae. Such eggs are 

 enveloped in stiff membranes, and it is therefore likely 

 that their symmetry is not inherent in their own structure 

 but is imposed upon them by the maternal cells which 

 produce the shell. In most other eggs, which are radially 

 symmetrical before fertilisation, it seems that bilaterality 

 is determined by the point at which the spermatozoon 

 enters ; this holds, for instance, for the sea-urchin and 

 frog. 



Experimental embryology. — Experiments on the power 

 of isolated blastomeres to produce complete dwarf embryos 

 have been made in almost all classes of animals. The 

 sea-urchin's egg is an example of the most " regulative '* 

 type : a blastomere representing only^ 73V of the egg has 

 been known to develop normally for some time, and it 

 seems possible that it is smallness of size rather than any 

 regional differentiation that prevents the perfect develop- 

 ment of these and smaller blastomeres. In most Coelenter- 

 ates the blastomeres are " totipotent/' i.e. capable of 

 producing a whole embryo, up to the four-cell stage, but 



