Biology 2e is designed to cover the scope and sequence requirements of a typical two-semester biology course for science majors. The text provides comprehensive coverage of foundational research and core biology concepts through an evolutionary lens. Biology includes rich features that engage students in scientific inquiry, highlight careers in the biological sciences, and offer everyday applications. The book also includes various types of practice and homework questions that help students understand—and apply—key concepts. The 2nd edition has been revised to incorporate clearer, more current, and more dynamic explanations, while maintaining the same organization as the first edition. Art and illustrations have been substantially improved, and the textbook features additional assessments and related resources.

By the end of this section, you will be able to do the following:

Discuss how fertilization occurs
Explain how the embryo forms from the zygote
Discuss the role of cleavage and gastrulation in animal development

By the end of this section, you will be able to do the following:

List the features that distinguish the kingdom Animalia from other kingdoms
Explain the processes of animal reproduction and embryonic development
Describe the roles that Hox genes play in development

Once sperm has entered the the oocyte, an ootid is formed. During early stages the ootid will contain male and female pronuclei along with the first and second polar bodies. Fusion of the male and female pronuclei will result in a single diploid nucleus or syngamy. Once syngamy has occurred, the zona pellucida then develops into an imprenetrable layer that prevents polyspermy and so polyploidy. Once the zona pellucida has developed, the ootid is now referred to as a zygote (diploid) and will begin undergoing mitotic divisions via a cleavage process that will begin to give rise to daughter cells called blastomeres. These cleavage divisions will begin to produce a 4-celled embryo and then an 8-celled embryo.