Section V 



CHAPTER 2 



Cleavage, Blastulation and Gastrulation 



DONALD P. COSTELLO 



The splitting of the egg into a multitude 

 of cells which build up the body of the 

 metazoon embryo and, eventually, of the 

 adult, is known as the cleavage or segmenta- 

 tion of the egg. Cleavage is the initial series 

 of mitotic cell divisions in the life history of 

 each individual following the egg stage, and 

 an infinitesimally small part of the immense 

 series of cell divisions that has no finite limits 

 in the past or in the future. If the activation 

 of the egg to development is looked upon 

 as the removal of a block to cell-division, 

 then each succeeding generation adds to a 

 continuum that extends back from the present 

 to the dawn of life. There is thus little 

 wonder that the early students of the subject, 

 including Kolliker, Remak, Virchow, Wilson 

 and others, saw in the division process the 

 fundamental basis of cell heredity. During 

 the cleavage period there is little or no in- 

 crease in total cytoplasmic mass, whereas, 

 typically, the rate of synthesis of nuclear 

 material is relatively high. 



Aside from this hereditary significance, 

 one of the major embryological problems is 

 whether cleavage is merely cell multiplica- 

 tion, by which the egg splits up into a num- 

 ber of equivalent or indifferent cells, or 

 whether the cleavage blastomeres have a def- 

 inite and predetermined pattern in relation 

 to each other and to the embryonic parts 

 for which they will contribute their cell 

 progeny. According to the former view, the 

 process of differentiation must follow cleav- 

 age, as a separate phenomenon; according to 

 the latter, cleavage and differentiation pro- 

 ceed concurrently and interrelatedly. His- 

 torically, the first attempts to settle this prob- 

 lem led to the type of study known as cell 

 lineage. 



Cell lineage is the tracing of the develop- 

 mental history of each individual blastomere 

 through to its idtimate fate in forming defini- 

 tive parts of the larva or adult. A synonym 

 is cytogeny. One may study the cell lineage 



of any animal, regardless of its pattern of 

 cleavage and type of differentiation. How- 

 ever, such studies have more than casual 

 significance in the case of those forms cleav- 

 ing with precise and predictable patterns, 

 and especially in those showing so-called 

 "determinate cleavage" (see below). 



There are three main categories of cleav- 

 age: radial, bilateral, and spiral. In as- 

 signing species to these categories, it is 

 necessary to bear in mind that one is not 

 dealing with fixed or rigid characteristics, 

 which persist throughout the cleavage pe- 

 riod. In spirally cleaving forms, there is 

 usually a transition to bilateral cleavage 

 following the earlier segmentation of the 

 embryo. 



Radial cleavage, well exemplified by the 

 cleavage of the sea urchin egg, occurs when 

 the successive cleavage planes cut straight 

 through the egg, at right angles to one 

 another and symmetrically disposed around 

 the polar axis. Thus, when the egg is viewed 

 from either pole, the blastomeres are ar- 

 ranged in a radially symmetrical form. Ra- 

 dial cleavage is found in the Porifera and 

 Cnidaria as well as among the echinoderms. 



In bilateral cleavage, the spindles and 

 cleavage planes are bilaterally arranged with 

 reference to a plane of symmetry which 

 coincides with the median plane of the em- 

 bryo. Styela and other tvmicates, Amphioxus, 

 the Amphibia and higher mammals clearly 

 show this type of segmentation. Richards 

 ('31) classifies the nematodes and rotifers 

 among the bilaterally cleaving forms but it 

 has been pointed out (Costello, '48a) that in 

 their early cleavages, these may actually be 

 variants of a spirally cleaving type. The 

 ctenophores show a type of bilateral cleav- 

 age which Richards has called disym- 

 metrical. 



Spiral cleavage is found in those forms in 

 which there is a rotational movement of cell 

 parts arovmd the egg axis, leading to a dis- 



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