60 PRINCIPLES OF EMBRYOLOGY 



(ii) Bilaterally symmetrical. In a great many eggs, the first two cleavages 

 are vertical, passing through the animal-vegetative axis, and cut off four 

 cells which are not equal, but arranged with a bilateral symmetry. This 

 is true, for instance, in some coelenterates (e.g. the ctenophore Beroe); 

 in ascidians and Amphioxus; and in most vertebrates where the accumula- 

 tion of yolk is not so large as to disturb the picture completely. 



(iii) Spirally symmetrical. There is a large group of invertebrates 

 (nemerteans, annelids, molluscs) in which the first two cleavages are not 

 quite vertical, but slightly inclined, so that when looked at from the 

 animal pole, the first four cells are arranged with a slight spiral twist. The 

 third cleavage plane is more or less horizontal, and cuts off a ring of 

 micromeres at the animal end; and these again do not lie immediately 

 above the lower ring of four cells, but are twisted out of place. The subse- 

 quent course of cleavage in these eggs has been studied in great detail, 

 particularly by a group of American authors at the begiiming of the 

 century (Conklin and E. B. Wilson are perhaps the best knovm of these). 

 It was shov^nti that the various cells formed after the first five or six 

 cleavages regularly develop into definite parts of the embryo, so that 

 the cleavage pattern is very intimately involved in the developmental 

 processes; we shall see (p. 62) that this is by no means usual in other types 

 of cleavage (Fig. 4.2). 



In many animals the simple spiral pattern is modified by a rather re- 

 markable process; just before the first division, the egg pushes out a large 

 pseudopodium-like excrescence, which is called the 'polar lobe' since it 

 forms near the vegetative pole (Fig, 6.4, p. 100). The cleavage runs so that 

 the whole of this lobe becomes incorporated into one of the two daughter- 

 cells. And the process is repeated through several of the later divisions. It 

 appears to be a mechanism for temporarily putting certain material on one 

 side. The cell into which this material eventually comes is the one from 

 which the mesoderm is developed, and thus one of the most important for 

 the future development. Moreover, it has recently been possible to cause an 

 egg with a polar lobe to cleave in such a way that the polar material is 

 divided among the first two cells; it was found that a double embryo 

 was formed (p. 99). This demonstrates the essential role which this mater- 

 ial plays in development, and enables us to understand why a special 

 mechanism has been evolved to keep it intact while it is being sorted out 

 into the fmal mesoderm-forming cell. The result also shows that the 

 spirally cleaving egg is not a true mosaic of parts whose fates are irrevoc- 

 ably fixed, since in the formation of such double embryos a good deal of 

 regulation must have been involved. 



(iv) Irregular cleavage. In some coelenterate eggs, the cleavage pattern 



