. Biological structure and function; proceedings. Biochemistry; Cytology. 500 T. GUSTAFSON It is for practical reasons suitable to denote the lowest part of the blastula (the region at the vegetal pole) as zone i, the next as zone 2, etc. These regions correspond to the presumptive; 1. primary mesenchyme 2. secondary mesenchyme 3. coelomic sacs 4. oesophagus 5. stomach 6. proctodoeum 7. ectoderm The films indicate that the morphogenetic activity in these zones can be partly reduced to similar types of cellular activities. One of these is a pulsatory activity of the cell surfaces bordering the
. Biological structure and function; proceedings. Biochemistry; Cytology. 500 T. GUSTAFSON It is for practical reasons suitable to denote the lowest part of the blastula (the region at the vegetal pole) as zone i, the next as zone 2, etc. These regions correspond to the presumptive; 1. primary mesenchyme 2. secondary mesenchyme 3. coelomic sacs 4. oesophagus 5. stomach 6. proctodoeum 7. ectoderm The films indicate that the morphogenetic activity in these zones can be partly reduced to similar types of cellular activities. One of these is a pulsatory activity of the cell surfaces bordering the blastocoel. The. Fig. 2. Further example of the primary invagination {a and h) and of secondary invagination (c). pulsations involve a centripetal translocation of cytoplasm, which causes the cells to become thicker at their centripetal than at their centrifugal ends. The pulsatory activity is often a forerunner of an emission of thin pseudopods (filopods) from the inner cell surfaces. These pseudopods either collapse or attach to the ectoderm and contract. A third change involves a decrease or increase in adhesion between the cells in the blastula or gastrula wall. These activities may be closely related to each other. It looks, at least, that a vigorous pulsation can be modified into a "shooting" out of pseudopods. I will try to give some examples how these three changes in cellular activity co-operate in the morphogenesis of the sea-urchin larva: A. The pulsatory activity of the cells and their resulting change in shape may bring about their release into the blastula cavity. This, how- ever, only occurs if their adhesion to each other and to the hyaline mem- brane, which covers the blastula surface, is low. The entrance of the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original IUB/IUBS Internation
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