KOFOID: DEVELOPMENT OF LIMAX, 107 
more rounded contour than the animal pole. The blastoporic invagi- 
nation now appears as a broad shallow depression, involving a large 
part of the median surface of the vegetative pole. It is deeper at its 
rounded anterior margin and gradually diminishes in depth posteriorly. 
Figures 43 and 44 (Plate VII.) give the appearance of the depression as 
seen from the surface at this stage. Figure 48 (Plate VIL) is a sagittal 
section through the lateral margin of the depression, and Figure 49 is a 
median sagittal section, the anterior end being at the left in both figures. 
Figures 45 and 46 (Plate VIL) represent respectively the posterior and 
anterior regions of tho blastoporie tract in transverse section. 
The dorsal surface also frequently shows a slight depression at this 
period. "This may be due to the temporary obliteration or reduction at 
this time of tho cleavage cavity, which is bounded laterally by the two 
bands of mesoderm (Plate VIII. Figs. 46, 48-50). The posterior end 
of the embryo is now slightly broader than the anterior, Figure 44 
(Plate VIIL). The broad ventral depression continues to deepen at 
the anterior end, and becomes narrower throughout its whole length. 
This results in a flattened embryo with an elongated median depression 
deepest at its anteriór end. 
Such flattened embryos with an elongated blastopore have been fig- 
ured by Lereboullet (62) and Lankester ('74) for Lymn:eus, and by 
Rabl (79) for Planorbis. Fol (80), who does not figure this stage of 
tho Pulmonate embryo, states that he has not found embryos so much 
flattened as those described by Lankester. The earlier writers upon 
Limax have passed over this stage in silence, though Lankester (775, 
Plate IX. Figs. 21, 22) figures two gastruleo of Limax agrestis, both of 
which appear to be of a later stage, showing considerable difference 
in size between the cells of the two outer layers. 
The anterior border of this deepening blastoporic trough becomes 
more abrupt, and the lateral borders more sharply marked out, while 
the depression of the posterior region is gradually obliterated. “The 
most marked change that accompanies this growth of the invagination 
is the increase in size of the anterior end of the embryo. Viewed from 
the ventral (Fig. 44, Plate VII.) or dorsal surface, it has had, up to 
5 
this time, a rounded quadrilateral outline, usually with the posterior end 
the larger ; but in the stages represented in Figures 51-53 (Plate VIIL.), 
the anterior end shows a marked increase in thickness as well as a 
lateral expansion. The thickening and lateral expansion of the anterior 
end are brought about by two influences, — the deepening invagination, 
and tho accumulation of mesoderm in the antero-lateral region of the 
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