234 



EARLY STAGES OF ARTHROPOD AND VERTEBRATE EMBRYOS. 



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Pf, 



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 ^- 5 - : ' ' /^k 



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FIG. 133. Diagrams, in mercator projection, illustrating the mode of growth of arachnid embryos during the 

 early stages; A, primitive cumulus, in the radiate or gastrula stages; B, appearance of the posterior cumulus and the 

 teloblasts, or the beginning of apical growth and the differentiation of the primitive head and trunk; C, appearance of 

 the medullary plate; the procephalic lobes; the marginal sense organs and ganglia; the telopore; and the segmenta- 

 tion, and lateral expansion, of the posterior portion of the germinal area; D, appearance of the thoracic appen- 

 dages; the growth of the palium over the procephalic lobes; the post-anal concrescence of the lateral plates of the 

 germinal area; and the forward extension of the vagal and anterior abdominal plates. 



gn. ncK.c.c. n.c. cj 



St. 



FIG. 134. 



FIG. 135. 



FIG. 134. Schematic cross-section of an arachnid embryo in the anterior thoracic region, showing the 

 relative positions and the mode of growth of the principal organs. 



FIG. 135. Schematic figure, in mercator projection, showing further concrescence of the germ wall, in a 

 hypothetical, large yolked embryo. The cephalic navel appears just in front of the procephalon, as the anlage of 

 the haemastoma, and the anterior appendicular arches are beginning to close in the haemal surface of the procephalon. 



not. 



ex.g. 



pc: 



FIG. 136. Section of a later stage 

 in the same region. 



FIG. 137. Section of a still later stage 

 in the branchial region. 



