288 VARIATION AND MONSTROSITIES. 



haemal organs lying in the axis of growth and the neural ones on the margins. 

 The way in which the new organs make their appearance is the reverse of that 

 by which organs disappear by median fusion and degeneration. 



7. Multiple embryos thus formed disappear again by median fusion and 

 anteroposterior degeneration. 



8. Variation in Limulus is primarily of a plus or minus nature. The endless 

 variety of results attained is due to the presence of a larger or smaller number of 

 organs. We are apparently always dealing with the same things and with the 

 same kind of stuff. When an unusual form of the aggregate appears, as in the 

 semicircular form of a half embryo, it is the indirect result of the absence of some 

 other part. In no case does a new organ or a new part appear that is different in 

 kind from those already existing ; in no case is an organ out of place in reference to 

 others; in all cases the organs come and go in a definite orderly sequence. 



The organs of the embryo, crystal-like, are always expressed in approximately 

 the same forms and in similar geometrical aggregates. The successive steps in 

 apical growth, in degeneration, in twin formation, and again in degeneration, are 

 minutely graded transitional phases in which the living substance in the egg of 

 Limulus finds formal expression. 



9. The parallel rise and fall, or the opening and closing of the checker-board 

 pattern, is a fundamental attribute of organic growth, and is a basic factor in the 

 morphogenesis of all segmented animals. The laws of differential apical growth, 

 and of the reverse process, or degeneration, are the keys to their morphology, 

 accounting for their bodily subdivisions, and for the unequal growth, specializa- 

 tion, and union of their various organs. 



