. Biological structure and function; proceedings. Biochemistry; Cytology. CELLULAR BASIS OF MORPHOGENESIS IN THE SEA URCHIN 5OI primary mesenchyme into the blastula cavity is an example of this activity, cf. Fig. I and [i, 6]. If the celkilar adhesiveness remains unchanged, on the other hand, the pulsatory activity and their change in shape bring about an invagination of the body wall. The first phase of invagination of the archenteron and the onset of the evagination of the coelomic sacs from the archenteron tip are examples of such a process, cf. Figs, i, 2 and [3, 6]. B. The pseudopod activ
. Biological structure and function; proceedings. Biochemistry; Cytology. CELLULAR BASIS OF MORPHOGENESIS IN THE SEA URCHIN 5OI primary mesenchyme into the blastula cavity is an example of this activity, cf. Fig. I and [i, 6]. If the celkilar adhesiveness remains unchanged, on the other hand, the pulsatory activity and their change in shape bring about an invagination of the body wall. The first phase of invagination of the archenteron and the onset of the evagination of the coelomic sacs from the archenteron tip are examples of such a process, cf. Figs, i, 2 and [3, 6]. B. The pseudopod activity brings about a strong extension of the invaginated or evaginated regions of the body wall. The second phase of invagination, cf. Figs, i, 2 and [i, 3], and the extension of the coelomic sac rudiments [3] are brought about by such a mechanism. The bending of. 8 hours Fi<i. 3. Diagram of the course of invagination of the archenteron in a larva of Psdnimechiiius miliaris (the larva in Fig. i). Inner height of the archenteron in microns is plotted against relative age in hours, the first stage arbitrarily called o hr. The pseudopodal activity is symbolized by the horizontal lines below the curve to the right, each line representing one visible pseudopodium; dashed line: pseudopodium intermittently visible; wavy line indicates direct contact between the secondary mesenchyme and the ectoderm. The schematic drawing to the left gives the appearance of the larva in three diflFerent stages corresponding to the marks below the abscissa. the archenteron tip towards the presumptive stomodaeum region of the ectoderm (Fig. 4 and [2]) is another result of pseudopod contractions. The contractions of the pseudopods will finally pull the pseudopodia-forming cells out from the archenteron tip and a so-called secondary mesenchyme is thus formed. The pseudopod activity finally brings about a rapid migration of the liberated primary as well as secondary mesenchyme cells. C. The role of a decre
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