1. The effect of chloroacetophenone (CAP) on the developmental stages of Planorbis (Indoplanorbis) exustus (Deshayes) has been studied.

  2. Treatment with the substance caused (a) exogastrulation, (b) inhibition of cytoplasmic and nuclear division of the big endoderm cells, (c) arrest of development accompanied by enlargement of blastomeres or delay in development, (d) a reversal of the flattening which normally follows cleavage, and (e) a secondary fusion of the micro- and megameres.

  3. The phase of development from the second cleavage to the fourth cleavage seemed to be CAP-susceptible and the period between the third cleavage and the third cleavage-flattened stages seemed to be critical from the point of view of production of exogastrulae. The exogastrulae which had developed for 77 hours showed some differentiation. The effect of CAP on the vegetative hemisphere was striking.

  4. The CAP-susceptible phase in general coincides with the process of ooplasmic segregation taking place in the cleaving egg, and the critical phase, in particular, coincides with the establishment of the -SH rich ectoplasm.

  5. The other abnormalities caused by the substance are interrelated and are explained on the basis of a probable –SH and –SS– balance existing in the cortical region of the blastomeres.

  6. The results have been interpreted to indicate that integrity of –SH groups is important in the morphogenesis of the species studied.

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