. Text-book of embryology. Embryology. VIII AETHEOPODA 181 of the blastodermic cells from a flattened to a more columnar shape. The cephalic lobes, which have increased in size, are brought nearer to each other so that they are only separated by a groove, and they are also approximated to the thoracico-abdominal rudiments. The endo- dermic disc is indented in its anterior portion by a deep, semicircular groove; this groove is the beginning of the process of gastrulation (Figs. 129 and 130), and may be termed the endodermic groove. The mesoderm which lies in front of this consists of a limited
. Text-book of embryology. Embryology. VIII AETHEOPODA 181 of the blastodermic cells from a flattened to a more columnar shape. The cephalic lobes, which have increased in size, are brought nearer to each other so that they are only separated by a groove, and they are also approximated to the thoracico-abdominal rudiments. The endo- dermic disc is indented in its anterior portion by a deep, semicircular groove; this groove is the beginning of the process of gastrulation (Figs. 129 and 130), and may be termed the endodermic groove. The mesoderm which lies in front of this consists of a limited number of large cells termed primary mesoderm, mingled with a larger number of small cells. The former will give rise to masses corresponding to the somites of Feripatus, from which the muscles and probably the genital organs arise; the latter constitute Eeichen- bach's so-called secondary mesoderm, they wander widely and occur every- where between ectoderm and endoderm, and appear to give rise to blood and connective tissue cells. Eeichenbach emphasizes the fact that these cells originate both from ecto- derm and from endoderm, but it seems probable that the primary mesoderm has an endodermic origin, while the secondary springs from the ectoderm. Soon the endodermic groove becomes a complete circle and the periphery of the endodermic disc is invaginated. Just as we Pk-. 129 have found to be the case in other eggs, the process of invagination can be analysed into (a) an in- . ^ ^ • crease in the number of cells and (b) an inwardly directed cytotaxis. The result of this kind of process is that the centre of the endo- dermic disc projects for a time as a kind of endodermic button, hut as the process continues this button is also carried inwards, and a circular blastopore is left where once there was a superficial disc of endoderm. The anterior part of the periphery undergoes the most rapid invagination, and so the endodermic sac projects forward beneath the thoracico-abdominal
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