Morphometrics of living specimens. A methodology for the quantitative three‐dimensional study of growing microscopic embryos

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Abstract
SUMMARY: A methodology is described which permits quantitative three‐dimensional studies of growing microscopic (and substantially transparent) animals to be made. The methodology has been applied in a limited way to the study of blastula formation in sand dollar embryos. A preliminary analysis has been made of the errors likely to arise (see Appendix).It was observed that the total blastomeric (cellular) volume decreased somewhat at the time of the second cleavage, and that the volume of the blastocoel simultaneously increased by about the same amount. It is suggested on the basis of this and other evidence that the blastocoel material is initially secreted by the blastomeres and that the blastocoel may subsequently innate, perhaps as a result of the imbibition of fluid from the extrablastular space. Further study will be required to confirm or invalidate this hypothesis.A study of plexiglas reconstructions suggests that formation of the blastular epithelium is the result of a coordinated pattern of cell cleavages and cell displacements. It is suggested that cell displacements may be the result of localized intracellular constrictions and/or changes in surface adhesion. Further study will also be required to confirm or invalidate this hypothesis.The methodology described in this paper is considered to be complementary to one previously described by Hou, Veress & Prothero (1970) for application to similar problems at the macroscopic level of resolution.