THE PRODUCTION OF NORMAL EMBRYOS BY ARTIFICIAL PARTHENOGENESIS IN THE ECHIUROID, URECHIS

¹ From the William G. Kerckhoff Laboratories of the Biological Sciences, California Institute of Technology, Pasadena

1. The eggs of Urechis may be activated by hypotonic solutions ranging from distilled water to 80 per cent sea water, and trochophores indistinguishable from those produced by fertilized eggs may be obtained.

2. The time of exposure necessary to bring about activation increases with increased concentration of sea water.

3. The activated eggs may extrude two, one or no polar bodies. When the exposure is such as to produce 100 per cent activation, two polar bodies are extruded on practically all the eggs, but to either side of this optimum exposure the proportion of activated eggs with no polar bodies increases. Eggs with one polar body occur in small numbers, and the single polar body is equal in size to the two polar bodies of the normal egg.

4. Only the eggs with no polar bodies divide and form embryos. Those with two polar bodies or with one polar body may sometimes produce uncleaved swimmers.

5. The eggs which extrude no polar bodies show a very poor response in other respects to the treatment. The eggs which extrude two polar bodies respond to the treatment in very much the same manner as if they had been fertilized. The fact that only the former cleave and develop is inconsistent with the view that for development to be obtained the artificially activated egg must behave like the fertilized egg in its initial response to the treatment.

6. The cleavage of the parthenogenetic egg is quite variable both in form and in time of division.

7. The normal embryos are produced in very small numbers. Even when normal-looking eggs in the eight-cell stage are isolated, less than 2 per cent normal development is obtained.

8. Among the abnormal embryos produced by the parthenogenetic ally activated eggs, a number of larvæ are found that may be classified in a radially symmetrical group. Such embryos may have the form of hollow blastulæ, early gastrulæ, or late gastrulæ, and their development ends without the appearance of bilateral symmetry.

9. The indentation of the unfertilized egg is found to mark the pole, and with respect to that point the polarity of the artificially activated egg is found to remain unchanged.

10. The first cleavage plane is found to pass through the entrance point of the sperm in 71 per cent of the cases. It may be concluded from this that the sperm is instrumental in determining the plane of bilateral symmetry.

11. It is suggested that in the absence of a localized external agent such as the sperm, it is a matter of chance whether the establishment of bilateral symmetry and other processes associated with the orientation of the embryo may be effected. This would account for the low percentage of normal development and the presence of radially symmetrical embryos.