(5 pts)  What is the fate of the first cells that involute behind the bottle cells into the blastopore as the blastopore lip forms on the future dorsal side of the embryo?  Describe very briefly what they will do during gastrulation and what they will have become by the end of gastrulation.



(8 pts)  We discussed in class the following transplantation experiment in Xenopus embryos.  From a recipient embryo, Gerhart and coworkers removed a ventral, vegetal blastomere at the 16-cell stage.  They replaced it with a dorsal vegetal blastomere dissected out of a donor embryo.  They observed that the recipient embryo developed a second, ectopic dorsal axis on the ventral side with a partial neural tube, notochord, etc.  They hypothesized that the transplanted cell induced dorsal structures to develop abnormally from the overlying, ventral, animal pole cells.  What is an alternative explanation?  What experiment did they have to make to confirm their hypothesis?



(2 points) Transcription of the embryonic genome begins only after many cleavages in some embryos (e.g. after the midblastula transition in amphibia). What does this imply about the origin of the majority of the RNA and protein that direct the initial events of development?

(5 points) What is the developmental significance of the control of cleavage plane and how is it accomplished?

(3 points) Distinguish between holoblastic and meroblastic cleavage.