Chapter 10, Problem 1RQ

During the embryonic period, the lungs first appear at about the

A. 10th day after fertilization

B. 24th day after fertilization

C. 6th week after fertilization

D. 12th week after fertilization

Summary Introduction

Introduction:

Reproduction is one of the basic functions in all living organisms. The process varies from the simple cellular division in case of unicellular bacteria to the very complex process of development of organs in higher animals. The development of lungs takes a special place in the development of the fetus. The development of lungs occurs in five phases—the embryonic phase, canalicular phase, pseudoglandular phase, alveolar phase, and terminal saccular phase.

Answer to Problem 1RQ

Correct answer:

6^th week after fertilization.

Explanation of Solution

Explanation for the correct answer:

Option (c) is given as “6^th week after fertilization.” The first phase of fetal lung development occurs in the embryonic period. The lung first appears as a small bud from the primitive esophagus. The lung bud is called the ventral respiratory diverticulum. This bud continues to grow and form the primitive bronchi, and by the end of the embryonic phase, the major bronchi—10 of them in the right lung and 9 in the left lung—are seen. Hence, option (C) is the correct option.

Explanation for the incorrect answers:

Option (a) is given as “10^th day after fertilization.” The embryo, which has just been fertilized and moved into the uterus, has to be implanted in the uterus for the pregnancy to be a successful one. This process of implantation of the embryo in the uterus happens around the 10^th day after fertilization. So, it is an incorrect option.

Option (b) is given as “24^th day after fertilization.” By now, the three layers, ectoderm, endoderm, and mesoderm, are formed. The amniotic sac enlarges by the 24th day. The traces of the digestive tract and the umbilical cord can be seen. So, it is an incorrect option.

Option (d) is given as “12^th week after fertilization.” The embryo begins to take a more human form at this stage. The eyes move closer and it is able to close its fist. The reflexes of the fetus also begin to develop. There is a rapid increase in weight as it marks the end of the first trimester. So, it is an incorrect option.

Hence, options (a), (b), and (d) are incorrect.

Conclusion

Hence, it can be concluded that lung development in the fetus begins by the 6^th week after fertilization. The first phase, which is called the embryonic phase, involves the emergence of the primitive lung in the form of a small bud. It grows and leads to the formation of primitive bronchi. The major bronchi are formed at the end of the embryonic phase.