Normal Plates of the Development of Lepidosiren paradoxa and Protopterus annectens. ■! 



limb being identical in appearance with the externa! gill rudiments of earlier stages and being at first 

 directly posterior to the external gills as if forming a member of the same series of organs. During the 

 later stages of development the most conspicuous features are the rapid growth of the true tail region, the 

 rapid growth in the head region in front of the main mass of yolk, and the spreading downwards of the 

 myotomes in the lateral wall of the body. With the increase in size of the head region the cement organ 

 becomes carried forwards on its ventral side so as eventually to lie well in front of the opercular opening. 

 The cement organ reaches a great size and forms a very conspicuous feature in the young Lepidosiren of 

 stages 32 — 34. After this atrophy gradually overtakes it and by stage 36 it has completely vanished. The 

 external gills reach their maximum about stages 32 — 34 in ProfojHerus, rather later (stage 34, 35) in Lepidosiren. 

 Thereafter reduction takes place, rapidly in Lepidosiren, very slowly in Protopterus where vestiges of the 

 three external gills frequently still persist in sexually nature individuals. In Lepidosiren there takes place 

 at stage 36 what may be called metamorphosis the external gills undergoing rapid atrophy, the cement 

 organ disappearing, the colouration of the young animal becoming much darker and the habits more lively. 

 Whether there is a similar concentration of developmental change in the case of Protopterus is not evident. 



The skin. (Graham Kerr, 1901 d.) 



Up to stage 32 — 35 in Lepidosiren the ectoderm retains its two-layered condition. Thereafter the 

 deep layer cells begin to multiply and the ectoderm gradually assumes the many layered condition of the 

 adult. At about the stage mentioned isolated cells begin to assume a clear glandulär character — the 

 forerunners of the great unicellular glands with which the adult epidermis is crowded, and flask glands 

 begin to appear as solid downgrowths of ectoderm which. develop a cavity secondarily. Chromatophores 

 which are already present in the mesenchyme show their strongly heliotropic tendencies by wandering 

 towards the surface of the body, crowding together immediately beneath the ectoderm and many of them 

 wandering in between the ectoderm cells from about stage 25 onwards. 



A remarkable local development of ectodermal gland cells goes to form the cement organ which is 

 so conspicuous a feature of the young Protopterus or Lepidosiren. Forming at its first appearance a long 

 crescentic structure spreading across the ventral side of the body, it becomes later on shortened from side 

 to side and is borne in the case of Lepidosiren on a conspicuous cushion-like structure. The glandulär part 

 of the cement organ arises as a thickening of the deep layer of the ectoderm, the cells of which become 

 tall and columnar, while the superficial layer soon breaks down so as to expose the ends of the gland cells. 

 The atrophy of the organ takes place in the usual way, the glandulär epithelium becoming penetrated by 

 vascular loops and invaded by crowds of leucocytes. 



Nervous system. (Graham Kerr, 1901 d.) 



The ectoderm over the whole archenteric region shows as early as stage 13 its deep layer slightly 



thickened its cells being somewhat columnar forming a wide medullary plate. Later on the axial part 



of this medullary plate shows active cell multiplication so that a deep solid keel is formed out of which 



the central nervous System develops. In both Lepidosiren and Protopterus the definitive cavity of the central 



nervous system appears as a secondary excavation in the at first solid rudiment. As the front end the 



rudiment becomes enlarged to form the brain ; it soon shows a segmentation into primary forebrain and 



rhombencephalon. It is not until a much later period that the primitive forebrain shows signs of division 



into thalamencephalon and mesencephalon. The hemispheres when they appear arise as paired and quite 



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