190-i.] NATURAL SCIENCES OF PHILADELPHIA. 217 



the endoderm-cells also contain masses of yolk {y-). Figs. 55 and 56, 

 endoderm-cells containing large masses of yolk, were taken from the 

 same embryo as Fig. 53. Fig. 57 shows a similar endoderm-cell from 

 a young planaiian one day old. This cell contains one of the large 

 vacuoles {v) characteristic of adult endoderm-cells, and the yolk is 

 much disintegrated. 



Thus it is perfectly plain, in this form at least, that the yolk-cells do 

 not serve as a "vicarious endoderm" (Metschnikoff) ; but endoderm- 

 cells, developed from the embryonic cells of the one genu laj'er, con- 

 sume the yolk-cells in the same manner as they do other food material 

 later on. 



Fig. 54 is a section from the head region of the same embryo as fig. 53, 

 showing brain (6) and eyes (e). By the fourth day after birth the yolk 

 has all disappeared from the lumen of the gut, but masses of it are still 

 to be seen in the endoderm-cells. The late embryos and young pla- 

 narians contain a very large proportion of embryonic cells and few 

 muscle- and gland-cells compared with mature animals. The tissue 

 of the whole body resembles that of recently regenerated parts of 

 adult planarians. It is interesting to note that the interval between 

 egg-laying and the developmejit of the permanent pharynx, eyes and 

 ner\ous system in the embryo is about the same as between merotomy 

 and regeneration of the same organs in pieces of adult planarians. 



The reproductive organs develop late, and as yet have been studied 

 in only two specimens. In one yoimg planarian, 8 weeks old, one ovary 

 was found, but no other reproductive or genital organs. In another, 

 10 weeks old, there was a small antrum with the penis just forming, 

 but no genital pore; one ovary and three testes were found. The 

 oocytes in these j^oung ovaries were small and only just distinguishable 

 from the cells of the yoke glands, which were quite well developed. 

 In the testes there were already mature spermatozoa. 



Summary. 



The points that seem to need especial emphasis are : 



1. In Planaria simplissima division of the chromosomes in both 

 maturation-divisions is longitudinl. 



2. The number of chromosomes in the maturation-divisions of the 

 germ-cells varies frrom 3 to 6, but is usually 3. 



3. In the embrj^ological development there is nothing corresponding 

 to the typical blastula and gastrula. After several segmentation 

 divisions the blastomeres form an irregular group embedded in a syn- 

 citial yolk-mass which forms a part of the embryo. Some of the blasto- 



