NOTES ON EMBRYOLOGY AND CLASSIFICATION. 4l7 



coelom. The second cause is the hereditarily accelerated 

 differentiation of the musculo-skeletal molecules. Just as 

 we saw reason to believe that the departure in the mode of 

 appearance of the enteric cell-layer in living organisms from 

 the mode in which it originated in ancestral forms was due 

 to PRECOCIOUS SEGREGATION^ SO here, again, we invoke this 

 principle, and find that it is capable of aflPording an explana- 

 tion of the most important and, at first sight, anomalous, 

 modifications of the primitive mode of development of the 

 musculo-skeletal cell-layer. 



4. Enteric origin of the calom. — The ancestral form pro- 

 vided with mouth and enteron, ectoderm and endoderm, and 

 a musculo-skeletal cell-layer delaminated from ectoderm 

 proceeded to develop diverticula of the enteron. In the 

 Zoophytes we find such diverticula running into the ten- 

 tacles or forming a periaxial cavity, (the axis being occupied 

 by the original enteron) or giving rise to periaxial or paraxial 

 canals. The ancestral form proceeded beyond this 

 to develop its enteric diverticula in the form of two large 

 outgrowths, a right and a left, which became shut ofi" from 

 the enteron by the pinching-in of the cells at the root of the 

 outgrowth ; and the two diverticula, or " parenteric growths'' 

 as we may call them, subsequently united to form one big 

 perienteric (perivisceral, peritoneal) cavity, the coelom. 

 This was the mode of origin of the coelom, the genetic 

 source of blood-vascular and lymphatic cavities and canals. 

 We find this mode of development of the coelom still main- 

 tained in many and widely difierent members of the animal 

 series, for instance, in Echinoderms, in Brachiopods, in 

 Sagitta. 



A very little change in this method of development 

 has given rise to the commonest mode of formation of 

 the coelom in existing animals. The outgrowth of the 

 enteron, or parenteric lobe, instead of being a hollow diver- 

 ticulum, is solid, and only develops its cavity after it has 

 become a considerable mass. Then it opens out or splits to 

 form the cavity or coelom, which by this retardation is pre- 

 vented from ever forming a part of the cavity of the original 

 archenteron. This modification of the ancestral mode of 

 formation of the coelom in the parenteric outgrowths is seen 

 in many Vermes (Kowalewsky, Oligochceta), in Arthropoda, 

 in some Mollusca, and most clearly and strikingly in Verte- 

 brata. I pointed out that this was the probable explanation 

 of the occurrence of the two kinds of coelom, called by Pro- 

 fessor Huxley ''an enterocoel" and " a schizocoel" respectively, 

 in this Journal, April 1875, p. 166. Professor Haeckel had 



