The philosophy of biology . nd of rolled-up condition, and that the processof development consisted merely in the unfolding (evolu-tion) of this embryonic organism, and in the increasein volume of its parts. This was the hypothesis ofpreformation held in the beginning of embryologicalscience. It involved various consequences: the limi-tation, for instance, of the duration of a species, sinceeach generation of female organisms contained in their THE VITAL IMPETUS 129 ovaries all the future generations ; with other conse-quences which the preformationists did not hesitateto accept. (2) The other
The philosophy of biology . nd of rolled-up condition, and that the processof development consisted merely in the unfolding (evolu-tion) of this embryonic organism, and in the increasein volume of its parts. This was the hypothesis ofpreformation held in the beginning of embryologicalscience. It involved various consequences: the limi-tation, for instance, of the duration of a species, sinceeach generation of female organisms contained in their THE VITAL IMPETUS 129 ovaries all the future generations ; with other conse-quences which the preformationists did not hesitateto accept. (2) The other view was the later oneof epigenesis : the egg was truly homogeneous andthe embryo grew from it. Obviously the acceptanceof this hypothesis led to vitalism, and we find that itwas abandoned just as soon as the embryologists recog-nised that physics provided a corpuscular theory ofmatter, when a return was made to the preformationviews of earlier times ; views which lent themselves tothe construction of a FlG. I2. mechanistic hypothesisof development. We may state verybriefly the main facts ofthe development of atypical animal ovum,such as that of the sea-urchin. The fertilised ovumdivides into two (2), and then each of these blastomeres divides again in a planeperpendicular to the first division plane (3) . The thirddivision plane is at right angles to the first two, and itcuts off a tier of smaller blastomeres from the tops of thefirst four. There are now (4) two tiers of blastomeres,a lower tier of large blastomeres and an upper tierof smaller ones. This is the 8-cell stage. Next,each of these blastomeres divides in two simultaneouslyso that the embryo now consists of sixteen cells. Afterthis the divisions proceed with less regularity, butafter about ten divisions the embryo consists of about1000 cells (210), and these are arranged to form ahollow sphere consisting of a single layer of cells. Thelatter are furnished with cilia, and the whole embryo, 130 T
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