Embryology and genetics / by Thomas Hunt Morgan.

  • Thomas Hunt Morgan
Date:
1934
Catalogue details

Licence: In copyright

Credit: Embryology and genetics / by Thomas Hunt Morgan. Source: Wellcome Collection.

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    outer surface of the single blastula, but there are two centers of anti- polar cells opposite each other at the equator. In Fig. 123a, the two halves are represented as separated along the polar axis, one is turned over and its outer surface placed inside the other half as in Fig. 123b. Both halves round up as shown in c. The partition is withdrawn, as in d, and a single hollow blastula results. The antipolar regions are on the same side of the blastula, but somewhat apart d-f. With the aid of these diagrams the behavior of united halves of the 16-cell stage is readily understood. If two halves are united, as in Fig. 121a, both typical and atypical plutei result. If two halves are united as in Fig. 121c, both typical and atypical plutei develop. If the reverse combination is made, as in Fig. 12 id, the development is quite irregular, Fig. 120b. A certain amount of regulation takes place, but the gastrulation is interfered with, and the plutei are abnormal with respect to the endoderm, Fig. 120b. When the eight cells of two antipolar hemispheres were united with a similar orientation as in Fig. i2ie, two invaginations oc- curred but the resulting plutei were very abnormal. When the micromeres are removed from each of two halves which are then united as in Fig. 12if, there is no primary mesenchyme formed, but a delayed secondary mesenchyme appears. A normal pluteus may develop. These grafting experiments show that the upper and the lower halves are already specific with respect to their future development. This was to be anticipated from the behavior of the polar and anti- polar halves when isolated, as well as from polar and antipolar halves of unsegmented eggs. There is some readjustment of the cells to each other, but on the whole the tendency to doubling, when two groups of macromeres (that give rise to the archenteron) are present, is evi- dent. To what extent the macromeres act as inductors for the rest of the embryo is not entirely evident. The evidence points rather in the opposite direction.