मानव भ्रूण में अस्थिकरण (Ossification) के प्राथमिक केंद्र आमतौर पर गर्भावस्था के किस सप्ताह के आसपास विकसित होने लगते हैं?

In which of the following embryonic stages of development does the blastocoel develop?

Which cell movement process is primarily responsible for initiating the formation of the primitive streak during avian and mammalian gastrulation?

During the embryonic development of a frog, the blastopore first appears during the: