The document describes the structure and development of three types of embryo sacs: 1) Monosporic embryo sacs develop from a single megaspore that undergoes three nuclear divisions without cell wall formation, resulting in an eight-nucleated sac with haploid nuclei. 2) Bisporic embryo sacs form when one cell of the megaspore dyad develops while the other degenerates, with each nucleus dividing twice to create the eight-nucleated sac. 3) Tetrasporic embryo sacs form when the four megaspore nuclei remain in a single cell (coenocyte) and all participate in embryo sac formation.

1. Structure & Types of embryo sac
B. Sc. Sem-III
Dr. Vasanta I. Kahalkar
Department of Botany
M. G. Arts, Science & Late N. P.
Commerce College, Armori

2. Development of Monosporic embryo
sac
• When out of four megaspore, only one forms embryo sac it is
called monosporic embryo sac. It was 1st observed in
polygonum. So, it is called polygonum type. The megaspore
nucleus divides by only karyokinensis (free nuclear divivsion).
Wall formation is delayed till eight nuclei are formed. The two
nuclei formed after the 1st mitotic division & move the
opposite poles where second & third mitotic division takes
place that result four nuclei each pole. One nucleus from each
pole move to central position. There after wall synthesis
occurs forming egg apparatus at the ( group of three cells )
micropylar end and antipodal cell ( group of three cells ) at the
chalzal end. One nucleus from each pole moves to the centre
of the embryo sac and fuses there forming the secondary
nucleus. This is the only diploid cell in the embryo sac, while
all other cell are haploid.

3. Development of Monosporic embryo sac

4. Development of Monosporic embryo
sac

5. Structure of embryo sac
• In typical embryo sac it is seven celled, eight
nucleated structure. At the micropylar end a group
of three cells are present called egg apparatus. It
is composed of one egg cell & two synergids.
Synergid are charaterized by the presence of
filiform apparatus that is made up of finger like
projection. At the chalzal end another group of
three cells are present called antipodal. One
nucleus from each pole moves to the centre of the
embryo sac and fuses there forming the secondary
nucleus. This is the only diploid cell in the
embryo sac, while all other cell are haploid.

6. Structure of embryo sac

7. Types of embryo sac
Type of embryo sac are distinguished by the
number of megaspores are involved in embryo
sac formation. Based on this criteria, three
types of embryo sac are classified. They are
1. Monosporic
2. Bisporic
3.Tetrasporic embryo sac

8. Monosporic embryo sac
• A monosporic embryo sac is derived from a
single haploid megaspor. Its nuclei divids thrice
to form 8 nuclei. All the nuclei of this embryo
sac are genetically alike. It is also called
Polygonum type.

9. 2. Bisporic embryo sac
• Bisporic embryo sac 1st division is accomplished by wall
formation so that a dyad is formed. One of the dyad cell
undergoes the 2nd division, where as the other one is degenerates.
• Both the nuclei of functional dyad cell take part in the formation
of embryo sac.
• Each nucleus undergoes two mitotic division, forming 8 nucleated
embryo sac.

10. 3. Tetrasporic embryo sac
• Sometime meiotic division of the megaspore mother cell is
not accompanied by cytokinesis and hence all the four
nuclei of coeno-megaspore single cell called coeno-
megaspore. All the four nuclei of coeno-megaspore
participated in the formation of embryo sac. This type
embryo sac is called tetrasporic embryo sac.

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