94 ` General Notes. \ : [Jan. 
posterior. The larger cell is next divided transversely, a smaller 
h ; 
mass being segmented from it behind. Then follows the division 
in twain of the smaller of the two primary tells. The four re- 
sulting blastomeres then assume a symmetrical disposition with 
respect to the future median axis. The three posterior smaller 
cells mark the future dorsal aspect of the body; the larger cell 
marks the position of the future anterior end. 
From this point onward the segmentation is essentially mero- 
laterally by a process of epiboly. Meanwhile, the posterior acu- 
minate end of the large anterior cell becomes segmented into a 
number of cells, which fake a share in the formation of the ecto- 
derm, together with the smaller dorsal cells already spoken of. 
While the formation of the entoderm is thus accomplished, the 
most anterior row of the dorsal group are destined, as shown by 
later events, to form the mesoderm. By this time the larger 
anterior cell has been further subdivided, and its component blas- 
tomeres fo the number of five, which form the rudiment of the 
endoderm, are included by the growth forward and downward 
of the advancing ectoderm. The mesodermic cells, which at first 
formed a transverse row at the edge of the dorsal group of ecto- 
dermic cells, are pushed farther forward and downward, and are 
finally thrust inward between the ectoderm and mesoderm along 
the anterior, or what may finally be regarded as the dorsal, border 
of the blastopore or prostoma. Since the mesoderm is developed 
in almost all bilateral forms from the entoderm, the development 
of it from the ectoderm in Rotifers, as here described, is probably 
characteristic and of taxonomic importance. A solid gastrula 
APETO pee aa) is thus formed, and the prostoma (blastopore) 
mes an anterior ventral position and marks the place where 
he permanent mouth is developed. The genesis of the meso- 
derm in Rotifers is contrasted with the mode of its origin in 
Astacus, according to Reichenbach, at the anterior margin of the 
blastopore. It is thought probable that the musculature and 
sexual organs are developed from the mesoderm 
The blastopore assumes a quadrate she and the ectoderm 
bounding it is divided into four well-mar lobes,—a right and 
left, an anterior and a posterior lobe. From on the invaginated por- 
tion of the ectoderm, lying within the blastopore, the cesophagus 
(which lies in front of the mastax) and the wheel-organ or trochal 
disk are developed. The posterior lobe of the ectoderm becomes 
divided off posteriorly from the blastopore by a transverse fissure 
Tone metamorphosis of the entode Didana mas mass ot cats A 
