Cleavage, Blastulation and Gastrulation 



215 



blast; the four basal cells, after producing 

 ectoblast and mesoblast, form entoderm, etc. 

 Superimposed upon these general aspects of 

 the ancestral reminiscences of cleavage are 

 numerous special features of the cleavage of 

 each species. These special features adapt 

 the cleavage of each form to the needs of 

 the future larva, as definitely as the larva 

 is adapted to the actvial conditions of its 

 environment. This principle of adaptation 

 in cleavage was enunciated by F. R. Lillie 

 (1893 and later papers). 



Almost every detail of the cleavage of the ovum 

 of Unio can be shown to possess some differential 

 significance. The first division is imequal. Why? 

 Because the anlage of the immense shell-gland is 

 found in one of the cells. The apical-pole cells divide 

 very slowly and irregularly, lagging behind the 

 other cells. Why? Because the formation of apical 

 organs is delayed to a late stage of development. 

 The second generation of ectomeres is composed of 

 very large cells. Why? Because they form early and 

 voluminous organs (larval mantle) . The left mem- 

 ber of this generation is larger than the right. Why? 

 Because it contains the larval mesoblast. The ento- 

 meres are very minute. Why, again? Because the 

 intestine remains rudimentary until a late stage; 

 thus a parallel instance to the apical-pole cells. One 

 can thus go over every detail of the cleavage, and 

 knowing the fate of the cells, can explain all the 

 irregularities and peculiarities exhibited. 



These peculiarities of cleavage are all due to the 

 precocious segregation of organs or tissues in sep- 

 arate blastomeres. The order and character of the 

 segregation again are ruled by the needs of the em- 

 bryo. Thus, one of its greatest needs is the large and 

 powerful shell with which it is provided. The neces- 

 sity of such provision being made has caused the 

 production of a large shell-gland, which has im- 

 pressed itself on the segmentation stages as the 

 largest of their blastomeres. I could illustrate the 

 principle in each of the cases just enumerated, but 

 will be satisfied with repeating the introductory 

 sentence of this paragraph in a more special form: 

 The peculiarities of the cleavage in Unio are but a 

 reflection of the structure of the glochidium, the 

 organization of which controls and moulds the nas- 

 cent material. (Lillie, 1895, p. 38.) 



The egg is not a cell dividing under the 

 stress of purely mechanical rules. It is "a 

 builder which lays one stone here, another 

 there, each of which is placed with refer- 

 ence to future development" (Lillie, 1895). 



In the theoretically simplest forms, the 

 divisions would be expected to be equal and 

 synchronous throughout the embryo. How- 

 ever, departures from this simplicity are 

 actually the rvde, in accordance with Lillie's 

 principle. In the egg of the annelid Nereis, 

 the typical succession of cleavages includes 

 the 2-, 4-, 8-, 16-, 20-, 23-, 29-, 32-, 37-, 

 38-, 41-, and 42-cell stages (see Fig. 69). 



Such deviations in synchrony are not de- 

 pendent upon the amount or distribution of 

 yolk or other substances in the eggs of ma- 

 rine invertebrates. Only in a very rough way 

 may the rule of Balfour (that the rate of 

 division in any region of the embryo is 

 inversely proportional to the amount of 

 deutoplasm it contains) be applied. In the 

 telolecithal ova of the salamander and cer- 

 tain teleosts, the slower rate of division in 

 the lower hemisphere appears to be corre- 

 lated with the greater qviantity of yolk 

 contained therein. However, remarkable dif- 

 ferences of tempo of cleavage are often shown 

 by blastomeres that display no appreciable 

 difference in yolk content. 



The inequality in size of the two daughter 

 blastomeres is likewise a phenomenon that 

 is inexplicable in terms of yolk content. 

 Among annelids, the first cleavage is un- 

 equal in Nereis, and the fifth is uneven in 

 Polygordivis. The first four blastomeres of 

 Crepidula and Patella (gastropods), Lepto- 

 plana (poly clad turbellarian), and Podarke 

 (annelid) are equal or near-equal in size. 

 Cumingia and Unio (lamellibranchs), 

 Ilyanassa (gastropod), Dentalium (scapho- 

 pod), and Amphitrite (annelid) show im- 

 equal early cleavages. The extent of the 

 inequality likewise varies among these ani- 

 mals. Treadwell (1899) attributes cleavage 

 inequality to the arrangement of segregated 

 materials within the cell in relation to the 

 direction of the cleavage plane, and points 

 out that this may be a quantitative segre- 

 gation rather than a qualitative one. Later 

 considerations of the mechanical causes of 

 inequality of cleavage take into account the 

 relation of polar lobe formation to the in- 

 equality of cleavage in Ilyanassa and Den- 

 talium, the inequality of the two poles of 

 the first cleavage spindle in Nereis, and the 

 combination of these two factors contributing 

 to unequal first cleavage of the egg of 

 Chaetopterus (Tyler, '30). In Nereis, the 

 difference between the centriolar poles and 

 asters of the first cleavage spindle is fore- 

 shadowed by an inequality of the poles of 

 the sperm diaster. Lillie ('09) attributes in- 

 equality of spindle poles to the molecular 

 structure of the ground substance, and a 

 more satisfactory explanation of the phe- 

 nomenon has not yet been suggested. How 

 an asymmetrical liquid crystal structure of 

 the ground substance could effect the pro- 

 duction of an asymmetrical pair of asters is 

 not clear. 



Since the cleavage furrow passes through 

 the middle of a cleavage spindle, the sizes 



