. Text-book of embryology. Embryology. PLATYHELMINTHES 109 rnes. mesectoderm in order to distinguish it from the parenchyma derived from ^.d? which occupies the more dorsal portion of the embryo, and which is termed mesoderm. The endoderm, with the exception of the two small cells budded from 4<^^ is all derived from the divisions of 4(^1. This cell, as we have seen, divides into right and left sisters; each then divides several times so that six or eight large cells are formed, lying at the posterior pole of the egg {end. Fig. 84). Soon afterwards the spreading edge of the ectoderm,
. Text-book of embryology. Embryology. PLATYHELMINTHES 109 rnes. mesectoderm in order to distinguish it from the parenchyma derived from ^.d? which occupies the more dorsal portion of the embryo, and which is termed mesoderm. The endoderm, with the exception of the two small cells budded from 4<^^ is all derived from the divisions of 4(^1. This cell, as we have seen, divides into right and left sisters; each then divides several times so that six or eight large cells are formed, lying at the posterior pole of the egg {end. Fig. 84). Soon afterwards the spreading edge of the ectoderm, consisting of the daughters of the second and third quartettes of micromeres, reaches the lower pole of the egg and covers this group of endoderm cells. During this time the other three micromeres of the fourth quartette have remained un- divided, but by the pressure of the daughters of ^(D- they are pressed higher up in the egg; their yolk-granules begin to coalesce so as to form fat-like drops, and these drops run together so as to form a limited number of enormous spherules. The nuclei remain large and conspicuous (N, Fig. 83). "When the ectoderm has completely covered the egg, an invagi- nation takes place at the lower pole; this is destined to form the stomo- daeum of the larva. The four minute "macromeres" become pushed in before it and ultimately disappear altogether. Then the mass of cells derived from ^d^ begins to separate one cell from another, in the middle, so as to give rise to a cavity, the future gut-cavity. As the endodermic cells multiply they spread in an amoeboid fashion over the outer surfaces of the yolk-spheres derived from 4a, 46, and 4c, so that these are included within the lumen of the alimentary canal and are absorbed (Fig. 86); But in Biscocoelis, according to Lang, 4« and 46 and 4c bud off small cells which take part in the formation- of the alimentary canal, so that in Flanocera one member of the quartette does the work which in Bi
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