216 



Embryogenesis: Preparatory Phases 



of the cells formed at a given cleavage are 

 determined by the position of the spindle at 

 the time. This has been admirably demon- 

 strated by the production of giant polar 

 bodies formed by divisions occurring while 

 a maturation spindle is being stretched and 

 displaced by centrifugal force (Conklin, '16; 

 Clement, '35). 



There are special inequalities which occur 

 characteristically during the cleavage pe- 

 riod in certain eggs, and independently of 

 the equal or unequal nature of the first 



Fig. 70. Normal Nereis trochophore about 20 

 hours after fertilization of the egg, viewed from the 

 left side, a.t.. Apical tuft; e.s., eye-spot; f.b., frontal 

 body; h.k., head-kidney; m., mouth; m.b., mesoblast 

 band; o.d., oil droplet in entodermal cell; p.a., anal 

 pigment area; pr., prototroch; s.d., derivatives of 

 the first somatoblast; St., stomodaeum. (From Cos- 

 tello, '45.) 



division. One almost universal case of cleav- 

 age inequality is the production of the first 

 quartet of micromeres at the third cleavage 

 of spirally cleaving eggs. These micromeres 

 are usually smaller than the basal macro- 

 meres, which fact accounts for the original 

 name designations. In Cerebratulus, how- 

 ever, the first quartet micromeres are 

 actually larger than the basal quartet macro- 

 meres. Another well known case of char- 

 acteristic unequal cleavage is that of turret 

 cell (trochoblast) production at the fourth 

 cleavage. These small cells in Nereis and 

 many other annelids produce the ciliated 

 prototroch (see Fig. 70). In Crepidula they 

 contribute to the ciliated velum. The 2d 

 and 4d cells are also smaller than their cell 

 mates. Another example is the separation 

 of the four micromeres at the vegetal pole 

 as the 16-cell stage is attained in the egg 

 of Arbacia and other echinoderms. Here, 



differences in the quality of the cytoplasmic 

 contents of the daughter cells are readily 

 apparent, since the micromeres are practi- 

 cally yolk- and pigment-free. This result is 

 brought about by a segregation of granules 

 preceding cleavage. No causal relation is 

 implied between the lack of granules in the 

 micromeres and the inequality of the cleav- 

 age by which these cells are produced. 



DETERMINATE AND INDETERMINATE 

 CLEAVAGE 



Eggs which cleave with a definite and 

 regular pattern are often called "deter- 

 minate" cleavage types; those in which the 

 cleavage pattern is subject to irregularities 

 and variations, and in which the position 

 of each furrow is not predictable, are called 

 "indeterminate." Obviously, these are not 

 absolute terms, but relative designations. 



The original meanings of these terms dif- 

 fered basically from this usage. Conklin ('97) 

 suggested that the determinate cleavage type 

 is that in which cleavage is typically con- 

 stant with respect to differentiation, as op- 

 posed to the indeterminate type, in which 

 cleavage may be changed without interfer- 

 ing with normal development. Synonyms 

 for these terms are, respectively, "mosaic" 

 eggs and "regulative" eggs. Assignment of 

 eggs to one or the other of these arbitrarv 

 categories is now recognized as vmiustified. 

 There are few, if any, animals in which the 

 normal cleavage pattern does not bear some 

 definite relation to axes, etc., of differentia- 

 tion, yet there are many animals in which 

 the cleavage pattern may be experimentally 

 altered (as by pressure) without interfering 

 with normal development. (See section on 

 redistribution of egg substances and their 

 relation to cleavage pattern.) 



CLEAVAGE WITHOUT NUCLEI 



Fragmentation of an enucleated portion 

 of an egg under the influence of an artificial 

 stimulus was apparently first recorded by 

 Delage ('99). This fragmentation, which was 

 described in connection with observations 

 on the non-nucleated cut portions of the 

 eggs of Lanice, Dentalium, and the sea 

 urchin, he considered to be a pseudosegmen- 

 tation of no special significance, since it did 

 not lead to the formation of a larva capable 

 of movement. 



Boveri ('18), McClendon ('08), and Fry 

 ('25) attempted to obtain development of 

 enucleated fragments by activating them 



