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before it leaves its mother, always contains germ-cells or developing 

 embryos. A study of these has led to the distinction of two very dif- 

 ferent lines of embryonic development in individuals of the same 

 species. In some of the individuals the germ-cells develop into so-cal- 

 led vermiform, in others into what are called infusoriform em- 

 bryos. Although it has been shown (Whitman) that the parents of 

 the two kinds of embryo are indistinguishable morphologically, it has, 

 nevertheless, been found convenient to give them different names. 

 The mother of vermiform embryos is colled a nematogen, the mother 

 of infusoriform embryos a rhombogen. 



The vermiform embryos arise in the entoderm cell of the nema- 

 togen in the following manner: In the earliest stages a single germ- 

 cell exists in the protoplasm of the axial cell. This germ-cell divides, 

 its daughter cells separate and divide in turn, and the process con- 

 tinues till the axial cell contains a number of isolated germ-cells. 

 Then, from some unknown cause, this method of division with subse- 

 quent separation of the cells changes abruptly. Each germ- cell divides 

 but the daughter cells henceforth remain together as blastomeres of 

 a regularly segmenting ovum. The cell-mass thus produced has been 

 interpreted as an epibolic gastrula with a single central cell as its en- 

 toderm. The spherical or elliptical cell-mass elongates after the spe- 

 cific adult number of cells is formed and we have the vermiform em- 

 bryo, which is a miniature of its mother. When mature it passes out 

 through the ectoderm cells of its parent into the renal cavity of the 

 Cephalopod. Sometimes the embryo does not leave its mother till it 

 contains within its own axial cell another vermiform embryo (van 

 Heneden), so that we may have a true case of emboîtement like 

 that seen in Gyrodactylus among the Trematodes. 



The offspring of the rhombogenic mother, viz. the infusoriform 

 embryos, are produced in a different manner, although they too start 

 from germ-cells like those to which the vermiform young can be 

 traced. In the rhombogen however, the germ-cells are few in number 

 and lie at intervals in the axial cell. Each of them begins to proliferate 

 by a regular cleavage and passes through an epibolic gastrula to a 

 stage resembling the very young nematogen and consisting of a single 

 large entoderm cell enveloped by a layer of ectoderm cells. At this 

 stage a sudden change occurs in the development, the very opposite of 

 that which supervenes in the development of the vermiform embryo. 

 The ectoderm cells free themselves gradually one by one and pass off 

 into the protoplasm of the axial cell in such a way that they form a 

 series on either side in the direction of the long axis of the mother. 

 While they are passing off as rounded isolated cells, the cell-mass 



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