ACEPHALIC AND ACAUDAL EMBRYOS. 261 



and the amputation of the head, in larval star-fishes, leaving only the posterior 

 part to complete its development. 



D. Final Stages of Degeneration. — The vast majority of all abnormal 

 embryos, whether single, doublets, or triplets, continue to degenerate by gradually 

 cutting off the more anterior segments, or by some modification of the process of 

 median fusion and antero-posterior degeneration. The details of the final stages 

 cannot be followed, but the general nature of the process and the final results are 

 readily observed. In the class of cases we shall now consider, after the disappear- 

 ance of all the appendages, the embryo may be reduced to a mere pit or sac, yet 

 preserving certain features, such as concrescing mesodermic areas and protruding 

 tail lobe, which show clearly the advanced stage of development in which the 

 whole embryo would have been, had no degeneration taken place. (Fig. 

 i86. J.K.) 



Some embryos may consist of two pits, or two groups of cells, like two 

 primitive cumuli, one corresponding to the head and the other to the tail end of the 

 body. (Fig. i86, G.H.) Finally these sac-like remnants are reduced to faint 

 clouds of scattered cells, or nuclei, which in turn disappear, leaving no trace of 

 living substance in the yolk. 



The conditions we have just described are important in that they give us a 

 glimpse of the negative processes of life, side by side with the positive ones. They 

 afford us a new picture of death, unlike the one with which we are most familiar. 

 In these embryos cell production, cell specialization, and cell decay proceed side 

 by side, for in every part of the body karyokinetic figures and the fragments of 

 decaying nuclei are found. The result depends on the relative intensity of these 

 three factors. The embryos apparently dwindle in size because the death rate 

 of the cells is greater than the birth rate. Nerve centers, sense organs and ap- 

 pendages disappear because the specialization of individual cells ceases and only 

 the simplest kinds remain. 



The process of degeneration is never exactly the same, but if completed it 

 invariably carries the organism back, in the main, over the old lines of progressive 

 development till it is reduced to its primitive condition, namely, a small community 

 of similiar, unspecialized cells which disappears with the death of the last survivor. 



This may be called the true natural death of an organism, all others are more 

 or less catastrophic, and are due to the increasing lack of coordination and of ad- 

 justment to the new conditions that have been created by growth. 



IV. Double Embryos. 



We may distinguish two kinds of fission, transverse and longitudinal. 



I. Transverse fission divides the embryo into anterior and posterior por- 

 tions, the point where the division most frequently occurs being between the third 

 and fourth thoracic appendages, or between the abdomen and thorax. The 

 steps leading up to this form have been described under the preceding sections. 



