98 LILLIE. 



EXPLANATION OF PLATE V. 



Fig. 59. Left side of embryo of over fifty cells, showing inclusion of Fand 

 formation oi y^ . 



Fig. 60. Superficial budding of the mesoblasts. 



Fig. 61. Horizontal optical section of stage with four cells of the shell-gland. 

 The larval mesoblast — Y — is passing into the segmentation cavity. Notice the 

 asymmetry of the primary mesoblasts — M. 



Fig. 62. Transverse section (actual) of a later stage, showing bilateral cleav- 

 age of Y. 



Fig. 63. Optical section in the same plane as Fig. 61. Divisions of iT/and Y. 



Fig. 64. View of gastrula from dorsal surface. The large cells are the 

 rudiment of the shell-gland. 



Fig. 65. Gastrula from lower pole. Entoderm in sepia. 



Figs. 66-67. Successive sagittal sections of the stage of Figs. 64, 65, and 72. 

 Fig. 66 passes a little to one side of the middle line ; Fig. 67 directly in the median 

 line. See line of section in Fig. 65. 



Fig. 68. Section of same stage through shell-gland and blastopore. See line 

 of section in F'ig. 65. 



Fig. 69. Median sagittal section after the invagination of the shell-gland. 



Fig. 70. Horizontal section of the same stage as Fig. 69. 



Fig. 71. The six cells from the region of the thread-gland. The central cell 

 is the protoblast of the gland. From the same stage as Fig. 69. 



Fig. 72. Gastrula from the side. 



Fig. 73. Median sagittal section of larva considerably younger than stage of 

 Fig- 79- 



Fig. 74. Part of Fig. 73 more highly magnified (Leitz Hom. oel-immers., yj). 

 Compare apical nucleus of thread-gland with nuclei on either side of aperture ; 

 compare, also, with Fig. 71. 



Figs. 75-78. Transverse sections from before-backward of the stage of Fig. 

 73. For region of sections, v. Fig. -jt^. 



Fig. 79. Young larva of Unio from the right side. Cf. apical nucleus with 

 ap. n. of Figs. 73 and 74. Extremely fine cilia of ventral plate not represented. 



Fig. 80. The same from in front. 



Fig. 81. Arrangement of nuclei (X 500) around opening of thread-gland of 

 Fig. 80. (C/. Fig. 71.) 



