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 VII.) is a sagittal 

 section through the lateral margin of the depression, and Figure 4'J is a 

 median sagittal section, the anterior end being at the left in both figures. 

 Figures 45 and 46 (Plate VII.) represent respectively the posterior and 

 anterior regions of the blastoporic 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 the 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 VIII.). 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 anterior end. 



Such flattened embryos with an elongated blastopore have been fig- 

 ured by Lereboullet ('62) and Lankester ('74) for Lymnaeus, and by 

 Eabl ('79) for Planorbis. Fol ('80), who does not figure this stage of 

 the 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 ('75, 

 Plate IX. Figs. 21, 22) figures two gastrulae 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 

 this time, a rounded quadrilateral outline, usually with the posterior end 

 the larger ; but in the stages represented in Figures 51-53 (Plate VIII.), 

 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 the accumulation of mesoderm in the antero-lateral region of the 



