. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. THE SPIRAL TYPE OF CLEAVAGE. 263. to be accomplished, an explanation appears possible. This divi- sion is a necessary step in the concrescence of the somatic plate. It does not bring " paratrochal material" into the proper posi- tion or into the proper degree of differentiation, but it does aid in accomplishing the end toward which each of the symmetrical divisions in the somatic plate contributes its part. In another case, , Amphitrite, where some of the preceding divisions are dif
. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. THE SPIRAL TYPE OF CLEAVAGE. 263. to be accomplished, an explanation appears possible. This divi- sion is a necessary step in the concrescence of the somatic plate. It does not bring " paratrochal material" into the proper posi- tion or into the proper degree of differentiation, but it does aid in accomplishing the end toward which each of the symmetrical divisions in the somatic plate contributes its part. In another case, , Amphitrite, where some of the preceding divisions are different and the material is somewhat differently grouped, no such division occurs, and the paratroch consists of four cells instead of six. The extreme precocious seg- regation, which extends back even to the first cleavage and in some forms [Arenicola) is indicated even before cleavage by the shape of the Qgg (Figs. 26, 27), is rendered possible by the fact that the whole cleav- age is strictly determinate in character. A determinate form of cleavage is, as I believe, the product of a considera- ble degree of modification in development in the direction of condensation. Thus the ap- pearance of precocious segre- gation in the earliest stages must indicate a still further departure from the original primitive forms of cleavage. Precocious segregation reaches its highest expression at present in the oligochaetes and the leeches, especially in the latter, where the development is very largely teloblastic, the material for almost the whole body being segregated into a few cells which form the germ bands. Here is not only segrega- tion, but regulated cell division in the highest degree. Teloblastic development must undoubtedly be regarded as the last term in a long series of modifications, all leading to a Fig. 26. —Arenicola. Egg before division, seen from upper Please note that these images are extracted from scanned page images that may have been digitally enhanced for r
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