The cell in development and inheritance . ne-cell stage, from the right side; /. girdle of prototrochal cellswhicli,give rise to the ciliated belt. the mechanical factor which predominates in the earlier stages; andin some cases, in the egg of Claveliua (Fig. 177) and other tuni-cates, this tendency predominates from the beginning. In boththese cases this tendency is obviously related to the growth-processto which the future bilateral embryo will owe its form ; ^ and everyattempt to explain the position of the cells and the direction of cleav-age must reckon with the morphogenic process t
The cell in development and inheritance . ne-cell stage, from the right side; /. girdle of prototrochal cellswhicli,give rise to the ciliated belt. the mechanical factor which predominates in the earlier stages; andin some cases, in the egg of Claveliua (Fig. 177) and other tuni-cates, this tendency predominates from the beginning. In boththese cases this tendency is obviously related to the growth-processto which the future bilateral embryo will owe its form ; ^ and everyattempt to explain the position of the cells and the direction of cleav-age must reckon with the morphogenic process taken as a blastomere is not merely a cell dividing under the stress of rude 1 Cf. Wilson C92, p. 444)- 370 CELL-DIVISION AND DEVELOPMENT mechanical conditions; it is beyond this a builder which lays onestone here, another there, each of which is placed with reference tofuture development. ^ The third class of modifications, due to unequal division of the cells,not only leads to the most extreme types of cleavage but also to its. C D Fig. 172. — The eight-cell stage of four different animals showing gradations in the inequality ofthe third cleavage. A. The leech Clepsine (Whitman). B. The chastopod Rhynchelmis (Vejdovsky). C. Thelamellibranch Unio (Lillie). D. Amphioxus. most difficult problems. Unequal divisions appear sooner or laterin all forms of cleavage, the perfect equality so long maintainedin Synapta being a rare phenomenon. The period at which the in-equality first appears varies greatly in different forms. In Polygordius(Fig. 170) the first marked inequality appears at the fifth cleavage; 1 Lillie, 95, p. 46. GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 371 in sea-urchins it appears at the fourth (Fig. 3); in AmpJiioxiis at thethird (Fig. 172); in the tunicate Claveliiia at the second (Fig. 177);in Nereis at the first division (Figs. 60, 171). The extent of the in-equahty varies in like manner. Taking the third cleavage as a type,we may trace every transition f
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectcells, bookyear1902
