The cell in development and inheritance . rther proof. ib) Axial Relations of the Primary Cleavage-planes. — Since theegg-axis is definitely related to the embryonic axes, and since thefirst two cleavage-planes pass through it, we may naturally look for adefinite relation between these planes and the embryonic axes; andif such a relation exists, then the first two or four blastomeres mustlikewise have a definite prospective value in the development. Such 380 CELL-DIVISION AND DEVELOPMENT relations have, in fact, been accurately determined in a large numberof cases. The first to call attention
The cell in development and inheritance . rther proof. ib) Axial Relations of the Primary Cleavage-planes. — Since theegg-axis is definitely related to the embryonic axes, and since thefirst two cleavage-planes pass through it, we may naturally look for adefinite relation between these planes and the embryonic axes; andif such a relation exists, then the first two or four blastomeres mustlikewise have a definite prospective value in the development. Such 380 CELL-DIVISION AND DEVELOPMENT relations have, in fact, been accurately determined in a large numberof cases. The first to call attention to such a relation seems to havebeen Newport (54), who discovered the remarkable fact that tJie firstcleavage-plane in the fi^og s egg coincides with the median plane of theadult body; that, in other words, one of the first two blastomeresgives rise to the left side of the body, the other to the right. Thisdiscovery, though long overlooked and, indeed, forgotten, was con-firmed more than thirty years later by Pfliiger and Roux (87). It. Fig. 177. — Bilateral cleavage of the tunicate Four-celled stage of Clavelifia, viewed from the ventral side. B. Sixteen-cell stage (VANBeneden and Julin). C. Cross-section through the gastrula stage (Castle) ; a. anterior;/. posterior end; /. left, r. right side. [Orientation according to Castle.] was placed beyond all question by a remarkable experiment by Roux(88), who succeeded in killing one of the blastomeres by puncturewith a heated needle, whereupon the uninjured cell gave rise to ahalf-body as if the embryo had been bisected down the middle line(Fig. 182). A similar result has been reached in a number of other animals byfollowing out the cell-lineage; by Van Beneden and Julin (84) PROMORPHOLOGICAL RELATIONS OF CLEAVAGE 381 in the ^^g of the tunicate Clavelina (Fig. 177), and by Watase (91)in the eggs of cephalopods (Fig. 178). In both these cases all theearly stages of cleavage show a beautiful bilateral symmetry, and no
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectcells, bookyear1902
