Essentials of Anatomy and Physiology

478

BOX21–1 TWINS

place, and each inner cell mass may develop into a

complete individual. Twins of this type may be

called monozygotic, meaning that they have come

from one fertilized egg. Identical twins are always of

the same sex, are very much alike in appearance,

and in other respects are genetically identical.

Conjoined twins are the result of the incomplete

separation of a splitting embryo (the splitting be-

lieved to occur 9 to 12 days after fertilization). The

name Siamese twins refers to Chang and Eng

(1811–1874), famous Chinese brothers born in

Siam, now Thailand. Conjoined twins may share

superficial parts such as skin and muscle, or they

may share a vital organ such as a liver. Advances in

microsurgery and reparative surgery have permit-

ted the separation of many sets of conjoined twins.

Fraternal twins are the result of two separate ova

fertilized by separate sperm. This may occur when

two ovarian follicles reach maturity and rupture at

the same time. Fraternal twins may be of the same

sex or different sexes. Even if of the same sex, how-

ever, they are as genetically different as any siblings

might be.

Identical twins are the result of the splitting of

the very early embryo before the cells start to

become specialized (usually within 8 days after fer-

tilization). For example, if a 16-cell stage becomes

separated into two groups of 8 cells each, each

group will usually continue to develop in the usual

way. Another possible cause is the development of

two inner cell masses within the blastocyst. This,

too, is before significant specialization has taken

Fertilization

2 cell stage

(36 hours)

4 cell stage

(48 hours)

Morula

Blastocyst Inner cell mass

Blastocoele

Trophoblast

Implantation

(7–8 days)

Embryonic disc

Endometrium

of uterus

Primary follicles

Ovary Maturing follicles

Ovulation

Ovum

Corpus luteum Corpusalbicans

Artery and

Fallopian tube vein

Sperm

Ovum (enlarged for

greater clarity)

Corona radiata

Zona

pellucida

Figure 21–2. Ovulation, fertilization, and early embryonic development. Fertilization

takes place in the fallopian tube, and the embryo has reached the blastocyst stage when it

becomes implanted in the endometrium of the uterus.

QUESTION:Find the blastocyst stage. What is the inner cell mass? What kind of cells make

it up?