. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. 142 BIOLOGICAL LECTURES. which will eventually become the ventral surface of the em- bryo, concomitantly with this concentration the outlines of the naupliar region of the embryo being formed. In Porcellio and Armadillidmm a similar concentration occurs, but in these forms the development of the naupliar region of the embryo is retarded, and one finds at an early stage a layer of cells closely aggregated together at one portion of the surface of the ovum, a few scattered cells being distributed o
. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. 142 BIOLOGICAL LECTURES. which will eventually become the ventral surface of the em- bryo, concomitantly with this concentration the outlines of the naupliar region of the embryo being formed. In Porcellio and Armadillidmm a similar concentration occurs, but in these forms the development of the naupliar region of the embryo is retarded, and one finds at an early stage a layer of cells closely aggregated together at one portion of the surface of the ovum, a few scattered cells being distributed over the rest of it. This aggregation of cells may be termed the blastoderm. To return now to the phenomenon to which I wish to call attention. At the conclusion of the second division of the nucleus in Porcellio, one finds that the peripheral protoplasm is no longer uniformly distributed over the surface of the ovum, but there has been a concentration of a large amount of it to one portion of the surface (Fig. 10). The nuclei are still imbedded in the yolk, and only after several divi- sions do they complete their cen- trifugal migration, entering the peripheral protoplasm and forming with it the blastoderm. At the period at which the concentration of the peripheral protoplasm occurs the nuclei are separated from it by equal and considerable distances, being united with it, however, by the protoplasmic network, and it is difficult to perceive how any of them could be able to influence the peripheral cytoplasm in such a way as to produce the con- centration. It seems rather that we have to do with an independent action of the cytoplasm, which precociously pre- pares for the formation of the blastoderm. We are now, I believe, in a position to appreciate the signif- icance of the peculiar direction of the spindle of the nucleus D of the ovum oijaera, a significance which is indicated by what has been said regarding the aggregation of the peripheral pro- toplasm of Porcellio. Both
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectbiology, bookyear1894
