214 



Embryogenesis: Preparatory Phases 



placement or inclination of the spindles with 

 respect to the symmetrically disposed radii. 

 It could have been called, and more properly, 

 oblique cleavage. Postcleavage rotations of 

 cell parts and blastomeres may also occur. 

 The rotational movements are characterized 

 by a regular alternation of direction, clock- 

 wise (right-handed, or dexiotropic) and 

 counterclockwise (left-handed, or laeotropic), 

 in successive cleavages. The inclination of 



in iour quadrants. In all forms, spiral cleav- 

 age becomes modified into bilateral cleavage 

 at some stage in development. The animal 

 types that may be added to the list of spirally 

 cleaving forms include the acoel Turbellaria 

 (which produce duets rather than quartets 

 of micromeres), the Cirripedia, as exempli- 

 fied by Lepas and Balanus (with cleavage 

 by "monets"), the Nematoda and probably 

 the Rotifer a. 



Fig. 69. The normal cleavage of the egg of Nereis (modified after Wilson, 1892). The circles in A, B and E 

 are oil droplets. The fertilization membrane has been omitted. A, Two-cell stage, polar view. B, Four-cell 

 stage, polar view. C, Eight-cell stage, polar view. D, Sixteen-cell stage, polar view. E, Sixteen-cell stage, 

 viewed from the right side. F, Twenty-two-cell stage, polar view. (From Costello, '45.) 



the spindles in spiral cleavage is usually ex- 

 quisitely clear at the third cleavage, at 

 which time the first quartet of micromeres 

 is formed. During the first and second divi- 

 sions, the obliquity of the spindles is less 

 clear, and these cleavages are sometimes 

 spoken of as prospectively spiral. In Crepi- 

 dula (Conklin, 1897), it is a postcleavage 

 rotation of daughter nuclei and of the proto- 

 plasmic areas in which these lie that marks 

 the first cleavage as prospectively spiral. 



Spiral cleavage (see Fig. 69) is generally 

 assumed to be limited to the polyclad Turbel- 

 laria, the Nemertea, the Annelida, and all 

 Mollusca except the Cephalopoda. Actually, 

 spiral cleavage is considerably more wide- 

 spread, if one considers certain modifications 

 of the typical oblique cleavage taking place 



The details of cleavage pattern for various 

 species may be found in MacBride ('14), 

 Da wy doff ('28), and other general texts, as 

 well as in countless original papers describ- 

 ing early development. It has been demon- 

 strated that there are resemblances nothing 

 short of marvelous between the cell lineage 

 of spirally cleaving eggs of even widely 

 separated taxonomic groups. In gastropods, 

 lamellibranchs, annelids and polyclad tur- 

 bellarians, the ectoderm of the embryo is 

 made up of three quartets of cells, formed 

 from the first four blastomeres; the fourth 

 product of the left posterior macromere con- 

 tains the mesoblast, except in the tvir- 

 bellarians. The turbellarian ectomesoblast 

 (derived from the second quartet) is repre- 

 sented in the other groups as larval meso- 



