. The Cambridge natural history. Zoology. DEVELOPMENT 227 these types are but the extreme terms of a contmuous series of forms which have all the same essential constitution and undergo the same metamorphosis. T]^e amphiblastula of Sycon raphanus (Fig. Ill) consists of an anterior half, formed of slender flagellated cells, and a posterior half, of which the cells are large, non-flagellate, and rounded. These two kinds of cell are arranged around a small internal cavity which is largely filled up with amoebocytes. The flagellated cells are invaginated into the dome of rounded cells during metam
. The Cambridge natural history. Zoology. DEVELOPMENT 227 these types are but the extreme terms of a contmuous series of forms which have all the same essential constitution and undergo the same metamorphosis. T]^e amphiblastula of Sycon raphanus (Fig. Ill) consists of an anterior half, formed of slender flagellated cells, and a posterior half, of which the cells are large, non-flagellate, and rounded. These two kinds of cell are arranged around a small internal cavity which is largely filled up with amoebocytes. The flagellated cells are invaginated into the dome of rounded cells during metamorphosis, in fact, become the choanocytes or gastral cells; the rounded cells, on the other hand, become the dermal cellsâan astonishing fact to any one acquainted only with Metazoan larvae. A typical parenchymula is that of Glathrina blanca (Fig. 112). When hatched it consists of a wall surround- ing a large central cavity and built up of flagellated cells interrupted at the hinder pole by two cells ()âthe mother-cells of archaeocytes. Before the metamorphosis, certain of the flagel- lated cells leave the wall and sink into the central cavity, and. undergoing Fig. 112.âMedian longitudinal , , 1 T n â section of pareuchyninla certain changes establish an inner mass ^^^^^ âf ciathrina Uanca. of future dermal cells. By subsequent ' p-g-c, Posterior granular â ^ , , cellsâarcnaeooyte motner- metamorphosis the remaining flagellated cells. (After Minciiin.) cells becorne internal, not this time by invagination, but by the included dermal cells breaking through the wall of the larva, and forming themselves into a layer at the outside. In the larva of C. Uanca, after a period of free-swimming existence, the same three elements are thus recognisable as in that of Sycon at the time of hatching; in the newly hatched larva of C. Uanca, however, one set of elements, the dermal cells, are not distinguishable. The difference, then, between the two newly hatched larvae is d
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
