DEVELOPMENT OF HALF-EGGS OF CHAETOPTERUS PERGAMENTACEUS WITH SPECIAL REFERENCE TO PARTHENOGENETIC MEROGONY

¹ From the Marine Biological Laboratory, Woods Hole, and the Biological Laboratory, Princeton University

1. The unfertilized Chaetopterus egg (94 µ) may be stratified and broken into unequal halves of a uniform size by centrifugal force. The nucleate white halves (83 µ) contain oil, clear layer, mitochondria and a little yolk. The non-nucleate yellow halves (64 µ) contain only yolk.

2. The centrifuged whole egg may develop, both fertilized and parthenogenetic, similarly to the normal uncentrifuged egg. Certain peculiarities have been noted.

3. The white halves, both fertilized and parthenogenetic, may cleave according to the typical annelid pattern. But there is a tendency toward equal first cleavage and amoeboid form and later, development with irregular cells and large nuclei; the blastulae tend to fuse together; these peculiarities are much more pronounced in the parthenogenetic than in the fertilized white halves.

4. The yellow halves, both fertilized (= fertilized merogone) and parthenogenetic (= parthenogenetic merogone) usually lift off a characteristic fluted fertilization membrane and pass through only one cleavage and become markedly amoeboid.

5. Early development without nuclei (parthenogenetic merogony) is established for the annelid, Chaetopterus, in addition to the five species of echinoderms previously studied. The development of the parthenogenetic merogones of Chaetopterus does not go as far as that of the sea urchin.