单选题 At the beginning of the gastrum, the inner cell mass divides into two layers, the one in contact with the blastocyst cavity being the hypoderm. As another layer

单选题 Mammals have an important change in sperm before they are fertilized. The location of this change is ().

单选题 Which is the time of the cleavage of the frog?

单选题 The gene encoding the（ ）transcription factor is transcribed in mouse forelimbs

单选题 Before the posterior portion of the tube has formed, the most anterior portion of the tube is undergoing drastic changes. In this region, the neural tube balloons into three primary vesicles ( )

单选题 The developmental basis of developmental biology is as follows：embryology、genetics、（ ）。

单选题 The ectoderm cells sink into the embryo to form a cord of solid cells,and then they create cavities to form（）

单选题 6.The following organs are not formed in the endoderm？