550 MEMOIRS OF THE NATIONAL ACADEMY OP SCIENUES. 



Plate XLIV. 

 (Stage YII.) 



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

 not yet diflferentiated from the skin. x234. 



Fig. 12!). Lateral longitudinal section through optic lobe and extremities of autenufe The differ- 

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

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



Fig. 130. Surface view of embryo of this stage, wich buds of four post-mandibular appendages 

 present The antenn* are covered with a hairy exuvium, which was probably 

 stripped ofi- from the anteunules in this preparation. The mouth is concealed by the 

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

 nearly at the extremity of the latter, which is slightly emargiuated. x 137 

 Fig. 131. Median longitudinal section in the series from which Fig. 129 was taken x '>34 

 Figs. 132-135. Serial transverse sections through embryo in similar stage. The germinal 'layers 

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

 period. An incomplete layer of elongated ceil.s (probably mesoblastic in origin com- 

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

 the neural thickening, from which the nervous system is in process of <levelopmont 

 In Fig. 134 rudimentary muscles suspend the stomoda^um 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 sernm, between the entoblastic lamella 

 and the ectoblast. x 234. 



BEFEKENCE LETTERS. 



A. I, first antenna. 



A. II, second antenna. 



Ah., thor.acico-abiloujinal fold. 



A. T.S., .alteration products of the yolk. 



Ed., ectoderm. 



End., endoderm. 



O. L., rudiment of optic ganglion. 



(H. A. II, antennular ganglion. 



Lb., labrnm. 



Mes., mesoderm. 



ifo., mouth. 



Mu., rudimentary muscles. 



Mx. I, first maxillary bud. 



O. E., retinal portion of optic lobe. 



S. O. G., brain. 



Sid., stomodfeuni. 



