THEORIES OF THE CCELOM 



It is, I think, now certain that they have nothing to do with the 

 formation of coelom. 



On the other hand, later researches, e.(j. those of Hatschek on 

 Polygordius (see Fig. 11), have confirmed the important view, 

 which I deduced from Kowalewsky's account of the origin of 

 the mesoblast in Lumbricus, namely, that the first rudiment of 

 the coelom, instead of detaching itself from the archenteron as 

 a pouch or even a solid mass of cells about to split, may separate 

 from the archenteric epithelium as a single pair of cells, which 

 take up their position in the blastocoel (space between ectoderm 

 and endoderm) in this state of naked simplicity (Fig. 11, A), and 



-n 



Fig. 11. — Transverse Sections of the Larva of the Ch^etopod Polvoordius to show 

 THE Origin of the Ccelomic Pouches from two Primary Cells detached from the 

 Archenteric Epithelium. 



A, section of an unsegmented larva, just in front of the anus, showing ec*, ectoderm, 

 end, endoderm, and mes, the two primary mother-cells of the coelom. /J, section of an 

 older larva near the tail, mrs, the cwloniic rudiments formed by the division and growth 

 of the primitive ca?lomic cells ; n, forecast of the nerve cowl. C, section of the same larva 

 nearer the head. The splanchnic (.■•7)) and somatic (so) walls of the cnelom have diverged 

 from one another forming; the coeloniic cavity. Ji, forecast of nerve cord. (Aft<"r Hatschek, 

 from Korschelt ami Ilcider.) 



then proceed to multiply so as to form a solid mass of cells right 

 and left (Fig. 11, B), and finally open out as two well-developed 

 ccelomic sacs (Fig, 11, C). This is a fine typical instance of 

 " precocious segregation," the original right and left ccelom cells 

 moving away from their proper and ancestral position in the 

 series of archenteric wall -cells at an astonishingly early period, 

 instead of waiting until they have formed a complete ctielomic 

 sac. 



The first important attack upon the theoretical identification 

 by Haeckel, Gegenbaur, and myself of the blood-space of Mollusca 

 with the coelom is due to the brothers Hertwig, who in their 



