A Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 



General Sketch of the Course of Development in Lepidoslren and Protopterus. 



External features. {Lepidosiren, Graham Kerr, 1900a; Protopterus, Budgett, 1901.) 



The egg of Lepidosiren (6,5—7 ^'^) and that of Protopterus (3,5—4 mm) undergo a complete but 



unequal segmentation. In Lepidosiren the first two furrows are meridional and they are succeeded by a series 



of four vertical furrows. These are liable to Variation and individual furrows may become actually latitudinal. 



In the case of Protopterus the early phases in segmentation have not yet been observed. 



Segmentation results in the formation of a blastula with a large segmentation cavity, roofed in by 



two layers of micromeres and overlying the heavily yolked macromeres which form the lower part of the egg. 



Gastrulation begins with the appearance of a latitudinal furrow (usually formed at first of a linear series 



of small depressions) about 8-10" below the equator of the egg and at first extending through it may be 



120" of longitude. The central part of this groove deepens to form the archenteron while its terminal parts 



flatten out and disappear so that by the time the blastopore is completed (the whole mass of macromeres 



being now covered in) it forms a short latitudinal opening-crescentic 



in shape and concave towards the dorsal side owing probably to the 



more active backgrowth of the central part of the dorsal lip as compared 



with its two ends. 



M • At a varying period about this time (stages 13—15) a slight flattening 



%DI> in front of the blastopore marks out the position of the medullary plate 



and by stage 16 definite medullary folds can be seen. For a short 



period — about stage 18 — varying with different eggs — the folds can 



be seen to be continuous with one another behind the blastopore or 



anus (cf. Text-fig. l). 



In the egg of stage 18 figured the bind end of the embryo has 

 Flg. I. Lepidosiren paradoxa. Stage l6+. 



X 8V2. View of specimen showing already proceeded too far in its development to show this feature. 



continuity of medullary folds behind t^, , j j . -i • ^ ■ ^^ ■ -n j^ i j • i.u 



,, -^ ihe head and tau project more prommently m Protojnerus and m the 



latter the body of the embryo extends round a considerably greater 



angular extent of the egg, the tips of the head and tail coming nearly into contact with one another at 



stage 25 in Protopterus while they remain widely separated in Lepidosiren. 



Conspicuous features in these stages are the cement organ — a crescentic structure on the ventral 

 side — and the external gill rudiments situated on visceral arches III— VI. About stage 23 in Lepidosiren 

 a vascular network is seen over the surface of the yolk, its cavities filled with colourless blood there being 

 for some time after its appearance no haemoglobin present. 



About the time of hatching (stage 27) the larva has a somewhat tadpole-like shape the hinder tail- 

 like region of the trunk being flexed ventrally. In Protopterus a larger proportion of the yolk is con- 

 centrated in the anterior swoUen part which as a consequence bulges more prominently and the rudiments 

 of various organs — external gills and eyes — are also seen to be relatively larger than in Lepidosiren. 

 Within a short space of time after hatching (stage 28) the trunk becomes straightened out and the pinnae 

 of the external gills appear as two rows of little knobs on their external surface. The myotomes are seen 

 to be growing actively in dorsiventral direction. 



About stage 30 active growth becomes apparent in the postcloacal or true caudal region which up 

 tili this time has been insignificant in size. Tlie limbs make their appearance about stage 31 the pectoral 



