A Contrìbut. of the Embryol., Life-history, sind Classificai, of the Dicyemitls. 87 



Fig. 58. Front view of the calotte, showing the alternate arrangement of the two 

 sets of iiolar cells. From a Nematogen .5mm long. Osmic acid. 



Fig. 59. Optical section of tlie head of a Ehombogen (.6mm long) from Hossia 

 macrosoma. Ectoderm charged with coarse shiuing granules of an 

 clongated angular form. Calotte remarkably large for this species. 



Piate 5. 



Figures ali from Dicyema moschatum where not othervvise stated. 

 Figs. (30 — 79 and 104 — lOtì magnified 1060/1 ; the remaining figs., 465/1. 

 Figs. 60, 61. A mature vermific germ-cell and a two-cell stage from Dicyema 



microcephalum. The germ-cell .009 mm in diam., its nucleus .006 mm. 



Acetic acid. 

 Figs. 62, 63, 64. A small germ-cell (.0015 mm), a mature germ-cell (.0085 mm) 



with fusiform striated nucleus , and another mature celi showing the 



uuclear spindle in transverse section. Acetic acid. 

 Figs. 65, 66. The first two cleavage-stages. Acetic acid. 

 Figs. 67, 68. Two views of the three-cell stage. Acetic acid. 

 Figs. 69, 70. Two four-cell stages. Acetic acid. 

 Fig. 71. A somewhat later condition of the four-cell stage. Three cells now form 



a cap to the fourth celi (endoderm). Acetic acid. 

 Fig. 72. A five-cell stage — four ectodermal and one endodermal celi (e). 

 V\g. 73. A six-cell stage from Dicyema microcephalum. Acetic acid. 

 Fig. 74. Optical section of a Castrala composed of 10 ectodermal cells and a cen- 

 trai endodermal celi. The endodermal celi has not divided siuce the 



four-cell stage. Acetic acid. 

 Fig. 75. An optical section of a Gastrula composed of 13 cells. Acetic acid. 

 Figs. 76, 77. An optical section and a surface view of a Gastrula. Blastopore 



nearly closed. Acetic acid. 

 Fig. 78. A little more advauced stage. The cells on one side of the blastopore 



multiply more rapidly than on the opposite side. Acetic acid. 

 Fig. 79. A sub-spherical embryo from Dicyemcnnea Eledones. The endodermal 



celi contains a single primary germ-cell. Acetic acid. 

 Figs. 80, 8] . An optical section and a surfjice view of the same pyriform embryo. 



The blastoporal side has an clongated pointed form. The broad end cor- 



responds to the future cephalic end. Acetic acid. 

 Figs. 82, 83. Optical sections of stili more advanced embryos. Osmic acid. 

 Figs. 84, 85. The nucleus of the endoderm in process of divisiou. Fig. 85 magni- 

 fied 1060/1. Acetic acid. 

 Figs. 86, 87. An optical section and a surface view of the embryo just after the 



first primary germ-cell has formed. Osmic acid. 

 Figs. 88, 89. Fig. 88 somewhat more advanced. It is about this time that the 



number of ectodermal cells becomes complete. Fig. 89 shows both of 



the primary germ-cells. Osmic acid. 

 Fig. 90. An embryo seen from one side. The polar and parapolar cells already 



distinguishable. Acetic acid. 

 Fig. 91. An embryo (.01 mm long) seen from the dorsal, or ventral side. Cilia 



already present. Osmic acid. 

 Fig. 92. A larger embryo with ali the cells outlined. Acetic acid. 

 Fig. 93. Head of a yuung individuai found free (.19 mm long). Osmic acid. 



