. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. Fig. 15. — Arenicola. matic plate, showing position of para- troch when first formed, x x, the first cells which meet in the median line during Since every cell plays a definite part, it is evident that morphogenetic cleav- age is necessarily accompanied by a certain degree of segregation, , the material which is to form the various organs is more or less completely segregated into definite cells or groups of cells, though they may not be dif- ferentiated, or, rather, the diffe
. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. Fig. 15. — Arenicola. matic plate, showing position of para- troch when first formed, x x, the first cells which meet in the median line during Since every cell plays a definite part, it is evident that morphogenetic cleav- age is necessarily accompanied by a certain degree of segregation, , the material which is to form the various organs is more or less completely segregated into definite cells or groups of cells, though they may not be dif- ferentiated, or, rather, the differentia- tion may be simply one of position. It appears probable that this bilat- erally symmetrical, morphogenetic cleavage has encroached upon the spiral period during the course of phylogenetic development and short- ened it, and that, in addition, inde- pendent modifications have arisen in the different groups, thus altering the relations of the constituent ele- ments. A study of the differences in cleavage in various forms brings to light a number of facts which appear to justify these conclusions. In the following paragraphs I have brought together some of the facts which appear to me to furnish evidence upon these points. I. The spiral period proper ends with different cell generations in — Arenkola. Portion of so- matic plate. The cells-r j: in contact ferent forms. in median line. Meager as are the data for a comparison between the different larger groups, they indicate what I believe will be amply demonstrated in the future, viz., that as the process of development becomes viore and viore highly modified, the strictly spiral ^^—Arenkoia Para- cleavage, when originallypresetit, gives zvay troch just before ventral ends _ _ are brought into contact by the at aji increasingly early stage to a tnorpho- concrescence of somatic plate. Fig. 16.—Arenicoia. Portion of somatic plate. Concrescence Please note that these images are extracted from
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