OF THE GERMINAL LAYERS IN GASTRORODA. 29 



Til fact it remains iincliaiiged until tlio cells enter into the i)er- 

 maneiit portion of the alimentary canal. Hence it in of great 

 use in determining the orientation of the l)ody. 



As to the mesoderm, its formation may he followed u]) to a cer- 

 tain stage with some exactness. ^Viter the eighth ectomere stage each 

 of the two daughter mesomere-cells m come to present the spindle. 

 It is directed obliquely forward and upward (Fig. 28), and appears 

 first most frequently in the left component (Fig. 29). The daughter- 

 cell or the first mesoderm-cell m^ is very small ; but its presence 

 is easily recognizable owing to its large nucleus (Fig. 30). The 

 second mesoderm-cell originates from the mesomere-cell as did the 

 first. The fact is shown in the horizontal section Fig. 32, and 

 the sagittal section Fig. 33. This, however, happens much later 

 ill time when there has already been much increase in the 

 numljer of the ectoderm-cells. The new mesoderm-cell in~z is 

 much like the first one but it takes a position more ventral than 

 the latter. The exact seat of these cells is clearly indicated in 

 Fig. 36, a sagittal section of the egg. The third mesoderm-cell 

 nh i« now derived from in^ beginning with the right component. 

 It is the most dorsally situated of all as is shown in Fig. 35, a 

 transverse section through the posterior side of the egg. The 

 first mesoderm-cell soon again subdivides, thus giving rise to the 

 formation of the fourth one ni^. Fig. 36 just mentioned above 

 also shows this fact, the S])indle in the first mesoderm-cell ii^ 

 being in the amphiaster stage. The mesoderm-cells, henceforth, 

 seem to segment rather rapidly with no apparent regular order. 

 At first they all aggregate as a mass in the posterior median 

 line of the 1)ody. Gradually, how^ever, they l)egin to spread toward 

 the lateral parts, where they soon arrange themselves in distinct 

 layers. In some other species of Mollusca the so called " larval 



