. Text-book of embryology. Embryology. Fig. 231.âView of the upper hemisphere of an embryo of Patella coerulea just before hatching. (After Wilson.) ap, apical cells ; , prototrochal cells. distinguishable from one another, all four constituting a mass of small cells covering the upper hemisphere of the egg (Fig. 231). The ectodermic cap of micromeres extends downwards so as almost to reach the vegetative pole by the process of growth known as epibole (see p. 72), and thus, by epibole, the process of gastrulation is completed. For a time a small opening persists at the vegetative pole, whe
. Text-book of embryology. Embryology. Fig. 231.âView of the upper hemisphere of an embryo of Patella coerulea just before hatching. (After Wilson.) ap, apical cells ; , prototrochal cells. distinguishable from one another, all four constituting a mass of small cells covering the upper hemisphere of the egg (Fig. 231). The ectodermic cap of micromeres extends downwards so as almost to reach the vegetative pole by the process of growth known as epibole (see p. 72), and thus, by epibole, the process of gastrulation is completed. For a time a small opening persists at the vegetative pole, where the macromeres are un- covered by micromeres, and this is the blastopore. In the meantime the mother cell of the meso- derm (4d) has become divided into right and left daughters. The proto- trochal girdle has now acquired the form of a complete circle through the acquisition of cilia by those cells which intervene between adjacent groups of primary trochoblasts. These cells may be termed secondary trochoblasts, and two of them in each quadrant are derived from the first quartette; they are in fact â s^ daughters of the outer cross cells Iqi^^. One secondary trochoblast in each quadrant is derived from the second quartette; these are the tip cells of the arms of the MoUuscan cross, 2q^i: and in this way the number of cells entering the prototrochal girdle is raised by three in each quadrant, making a total of 28 (16 + 12). Occasionally the tip cell divides into two, and then there are 32 second- ary trochoblasts. The cilia borne by the secondary trocho- blasts are at first much shorter than those borne by the primary trochoblasts (Fig. 232). By secondary shiftings these cells become rearranged so as to constitute a complete girdle of powerful ciHa, behind which is an equally complete girdle of smaller Fig. 232.âLateral view of a late embryo ot Patella coerulea showing the mode of completion of the prototrochal girdle. (After Wilson, altered.) , primary trochobl
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