. The Biological bulletin. Biology; Zoology; Marine biology. 326 SVEN HORSTADIUS relation between these asymmetries and a supposed bilateral organiza- tion of the egg. D. Isolation of the Four Animal and the Four Vegetative Cells of the Eight-Cell Stage Zeleny (1904) found that larvae developing from the upper quartet have an apical organ, but no archenteron, those from the lower quartet have an archenteron, but no apical organ, while those from lateral four-cell groups have both apical organ and archenteron. Zeleny drew the conclusion that certain organ-forming materials are definitely separa
. The Biological bulletin. Biology; Zoology; Marine biology. 326 SVEN HORSTADIUS relation between these asymmetries and a supposed bilateral organiza- tion of the egg. D. Isolation of the Four Animal and the Four Vegetative Cells of the Eight-Cell Stage Zeleny (1904) found that larvae developing from the upper quartet have an apical organ, but no archenteron, those from the lower quartet have an archenteron, but no apical organ, while those from lateral four-cell groups have both apical organ and archenteron. Zeleny drew the conclusion that certain organ-forming materials are definitely separated by the third furrow and that the larvae from the upper and the lower quartet have not the power of replacing the material lacking. The meridional halves, however .^possess both kinds. A B C D Fig. 5. Larvae from the isolated four animal cells of the 8-cell stage. of material. Zeleny preserved his larvae before they were fully differ- entiated. Yatsu (1910&) isolated 7 animal quartets, six of which gave a blastula with apical organ, with 1 to 3 flagella, and, to judge from the drawings, a ciliated field or band at the opposite pole. Thus no gastrulation occurred. Strange to say, the seventh larva developed into an almost perfect pilidium (Yatsu, 19106, Fig. 18, D). Of the 3 vegetative quartets Yatsu mentions two as "; The third is an almost normal pilidium {loc. cit., Fig. 18, /). My material numbers 10 pairs of animal and vegetative quartets, and moreover 9 animal and 4 vegetative fragments of the same kind, but of which the corresponding vegetative and animal quartet respec- tively are missing. None of the 19 animal fragments gastrulated. They gave a blastula with a pretrochal pavement epithelium, a thickened vegetative ciliated band or field, and one, or several, apical. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may no
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