FIG. 200 



Figs, 200-215. — Development of lamprey, Petromyzon planer i. Figs. 200-204, 208-212 

 X 18, others X about 30. 200, 201. First cleavage, beginning and concluded. 202. Third 

 cleavage. 203. Fourth cleavage, in section, showing beginning of segmentation cavity. 204, 

 205. Early and late blastulae, in section. 206, 207. Early and late gastrulas, in section. 208, 

 210, 212. Early embryos showing growth of head end. 209, 211. Sagittal sections of early 

 embryos showing differentiation of organs. 213, 214. Transverse sections of early embryos. 

 215. Sagittal section of newly hatched larva, Ammoc<ztes. (Figs. 211, 215, after GOETl'E, others 

 after V. Kupffer.) 



BP. Blastopore. C. Coelenteron. CH. Notochord. DL. Dorsal lip of blastopore. EC. 

 Ectoderm. EN. Entoderm. EP. Epiphysis. G. Gut. H. Heart. Af. Central nervous 

 system. MES. Mesoblast. N. Nasal pit. NC. Neurenteric region. S. Mouth pit, stomo- 

 daeum. SC. Segmentation cavity. T. Thyroid gland. Y. Yolk and yolk cells. 



189 



