Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 GEPHYREA. 393 The cerebral ganglion, cesophageal ring and ventral cord run inside the dermal muscular tunic. Only one pair of segmental organs, known as brown tubes or ventral glands, is present. The blood vascular system is well developed. Development.—The segmentation is com- plete and is followed by the formation of a gastrula by invagination. The blastopore marks the ventral side. The two posterior marginal cells* of the entoderm move in- wards as primitive mesoderm cells,
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 GEPHYREA. 393 The cerebral ganglion, cesophageal ring and ventral cord run inside the dermal muscular tunic. Only one pair of segmental organs, known as brown tubes or ventral glands, is present. The blood vascular system is well developed. Development.—The segmentation is com- plete and is followed by the formation of a gastrula by invagination. The blastopore marks the ventral side. The two posterior marginal cells* of the entoderm move in- wards as primitive mesoderm cells, and give rise to the mesoblastic bands which do not undergo segmentation. Invaginations of the ectoderm of the animal pole and ventral sur- face of the em- bryo give rise to cepha- lic and ventral p late s respec- tively, while the remain- der o f the ecto- d e r m cells grow round thes-e and form an external envelope for the embryo of the nature of a serous mem- brane (serosa). Cilia project from the latter through the pores of the vitelline membrane and are employed by the embryo in
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