550 MEMOIRS OP THE NATIONAL ACADEMY OF SCIENCES. 



PLATE XLIV. 

 v (Stage VII.) 



FIG. 128. Transverse section through embryo, in the region of the first maxilla. Nervous system 

 not yet differentiated from the skin. x234. 



FIG. 123. Lateral longitudinal section through optic lobe and extremities of antenna;. The differ- 

 entiation between the retinal and ganglionic parts of the eye is very clear. Meso- 

 blastic cells, representing rudimentary muscles, are seen attached to the body wall. 

 x234. 



FIG. 130. Surface view of embryo of this stage, with buds of four post-ruandibular appendages 

 present. The antenna- are covered with a hairy exuvium, which was probably 

 stripped off from the anteumiles in this preparation. The mouth is concealed by the 

 labriim, which nearly meets the thoracico-abdominal fold. The anus is situated 

 nearly at the extremity of the latter, which is slightly emargiiiated. x!37. 



FIG. 131. Median longitudinal section in the series from which Fig. 129 was taken. x234. 



FIGS. 132-135. Serial transverse sections through embryo in similar stage. The germinal layers, 

 definitely established in the egg-nauplius stage, are clearly differentiated at this 

 period. An incomplete layer of elongated cells (probably mesoblastic in origin, com- 

 ing from wandering yolk cells), Mes., Figs. 131, 132, is seen between the yolk and 

 the neural thickening, from which the nervous system is iu process of development. 

 In Fig. 134 rudimentary muscles suspend the stomodseum to the body wall, and in 

 Figs. 133 and 135 the three germinal layers can be clearly seen. The heart is repre- 

 sented by the space filled with mesoblast and serum, between the entoblastic lamella 

 and the ectoblast. x234. 



REFERENCE LETTERS. 



A. I, first antenna. 



A. II, second autenna. 



Al., thoracico-abdominal fold. 



A. r. ,$., alteration products of the yolk. 



Ect., ectoderm. 



End., endoderui. 



';. f.., rudiment of optic ganglion. 



(it, A. II, autennular ganglion. 



Lt>., labrnm. 



Mes., inesoderm. 



.Vo. , mouth. 



.)/., rudimentary muscles. 



MX. I, first maxillary bud. 



U. E., retinal portion of optic lobe. 



S. 0. G., brain. 



Std., .stoniodremn. 



