The middle cell by a series of divisions gives rise to the 

 whole embryo, including the stem, hypocotyl and root tip. 

 The stem tip arises laterally in a notch in the side of the 

 embryo. 



(e) — Development of the Embryo in Animals 



The fertilized egg-cell divides rapidly by segmentation 

 forming a mass of cells, but the nature of segmentation 

 differs in different groups of animals according to the 

 amount and distribution of yolk present. In mammals 

 total segmentation occurs and the cells are nearly equal in 

 size; in frogs and toads also total segmentation occurs but 

 the cells are unequal on account of the large amount of yolk. 

 In fishes, birds, reptiles, partial segmentation occurs on 

 account of the excessive amount of yolk, and only a small 

 disc-shaped mass of protoplasm lying on top of the yolk 

 segments; in insects and crayfish there is also partial seg- 

 mentation but the segmentating mass of protoplasm is 

 peripheral, surrounding the central yolk. This stage of 

 segmentation is called the blastula; the second stage, the 

 gastrula, is formed by an inpushing or infolding of the 

 blastoderm layer so that a body with two layers is produced 

 — the ectoderm and the endoderm layers. Soon a third layer, 

 the mesoderm, is formed between these two, chiefly from 

 the endoderm. From these three layers all the tissues of 

 the body are formed. From the ectoderm arise the sen se 

 o rgans, nervous system," and outer integumen t; f rom th e 

 e nHoderm the digestive portio n_ofthe^ nmentarv tract, and 

 frpm the mesoderm the skeleton^ muscles, blood and lymp h 

 s ystems, reproQuctive org an s and connective tissue s. The 

 nrianner ot tormmg the ammal body is different for the diff- 

 erent groups, but in the vertebrates "the ectoderm forms 

 first a shallow groove along the axis of the embryo, which 

 becomes deeper by the more rapid growth of the sides, until 

 the latter closes over and meets above to form a tube, the 

 anterior end of which, by a further complication of folds, 

 flexures and cell growth becomes the embryonic brain" 

 (Abbott). 



With regard to the segregation of the germ cells Weis- 

 mann says that in one group of animals segregation takes 

 place at the very beginning, and in the other group it does 

 not occur until a later period. In Ascaris and m the Dip- 

 tera among insects the fertilized ovum divides into two 

 cells, one giving rise to the whole body (soma) and the other 

 only to the germ cells lying in this body. In all other 



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