Bio1331 Animals - Evolution - Overview Of Body Plans (Lecture Two)

At gastrulation, both protostomes and deuterostomes develop a coelom, which is a fluid-filled cavity completely lined by mesoderm. The coelom plays a crucial role in providing space for organ development, allowing for movement and flexibility, and acting as a hydrostatic skeleton in some organisms. This cavity is essential for the development and functioning of various organ systems in both protostomes and deuterostomes.

During gastrulation, the hollow blastula undergoes a process where cells start to indent and rearrange, resulting in the formation of a blastopore. The blastopore is the opening that forms in the developing embryo, which eventually gives rise to the digestive tract. This process is crucial for the formation of the three germ layers and the establishment of the body plan in many multicellular organisms.

Cleavage is the first few divisions of a zygote. During cleavage, the zygote undergoes rapid cell division without significant growth, resulting in the formation of smaller cells called blastomeres. These blastomeres continue to divide and eventually form a solid ball of cells called a morula. Cleavage is an essential process in embryonic development as it establishes the basic body plan and determines the number of cells in the developing embryo.

Determinate cleavage is a type of cell division in which each blastomere, or daughter cell, possesses different transcriptional regulators/factors that determine its specification and fate from an early stage. Transcriptional refers to the process of gene expression, where genetic information is transcribed from DNA to RNA. This suggests that the differentiation and fate of each blastomere in determinate cleavage are determined by the specific transcriptional regulators and factors present in each cell. Transcription, on the other hand, refers to the general process of gene expression and does not specifically address the regulation and specification of individual blastomeres.

Asymmetry refers to the lack of symmetry or balance in an organism's body structure. Sponges are primitive multicellular organisms that lack any definite body shape or symmetry. They have irregular and asymmetrical body plans, with no distinct front or back, left or right sides. Therefore, it is correct to say that asymmetry occurs in sponges only.

In spiral cleavage, mitotic spindles rotate clockwise at the 3rd division. This means that during cell division, the spindles, which are responsible for separating the chromosomes, rotate in a clockwise direction. This rotation pattern is specific to the 3rd division in spiral cleavage, indicating the direction of cell division and the formation of different cell layers.

Spiral cleavage allows for the development of asymmetry.