CHAPTER V 



ECHINODERMS 



I. Normal development 



The early stages in the development of echinoderm eggs are very 

 simple, and provide classical examples of the two fundamental forms, the 

 blastula and gastrula. Moreover, the physiology of these stages has been 

 rather fully investigated. There is therefore much of interest in echino- 

 derm development even for a general account of embryological principles, 

 in spite of the fact that the later stages are highly complex. The simple, early 

 stages of development lead to the formation, not of the adult, but of a 

 larva, which is usually the so-called pluteus'. The insertion of a larval 

 stage into the life-history is, of course, very common among invertebrates. 

 Such larvae fulfd many functions; they may, for instance, faciUtate the 

 dispersal of the species, as in many groups of parasites; or they may seem 

 to have been evolved as the quickest way in which the egg can be con- 

 verted into an animal capable of feeding itself The latter would appear 

 to be the raison d'etre of the Pluteus; it is a httle animal which can swim 

 by means of cilia, and feed itself from the minute life among the plankton. 

 It is only after a considerable independent existence that it becomes con- 

 verted, by a complicated metamorphosis, into the adult. In this discussion, 

 we shall not attempt to deal with anytliing more than the first steps of 

 development, by which the larva is produced (Reviews: Lehmann 1945, 

 Horstadius 1939, 1949)- 



The cleavage of the echinoderm egg is total, and radially symmetrical. 

 The first two cleavages are vertical, through the animal pole. The third 

 is horizontal, running slightly above the equator, so that the upper four 

 cells are rather smaller than the lower four. From this point onwards, the 

 cleavages in the animal and vegetative halves take different courses. At 

 the fourth cleavage, the animal cells divide into a flat ring of eight, while 

 the vegetative ones cleave very unequally into two rings, four large 

 'macromeres' above and four tiny 'micromeres' below; the cleavage 

 planes are thus nearly at right angles in the animal and vegetative halves. 

 The same is true at the next cleavage, but here it is in the animal half that 

 the division is horizontal, while in the vegetative half it is more or less 

 vertical. At the sixth cleavage, all the division planes are horizontal, and 

 thus we come to a stage with four rings of animal cells, two rings of 

 macromeres, and two of micromeres, there being eight cells in each ring. 



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