STAGES OF DEVELOPMENT 143 



transverse sections cut behind the level of the seventh mesoderm segment. The organ 

 originates as a ridge of mesodermal cells between the mesoderm segments and the lateral 

 mesoderm. The latter splits into the splanchnic and somatic layers and, at this stage, is 

 united to the mesoderm segments by a narrow isthmus of cells. The excretory ridge 

 first makes its appearance just dorsal to this isthmus. Posteriorly it is a solid rod of cells, 

 but at its anterior end it acquires a cavity foreshadowing that of the archinephric duct. 

 The amnion has grown over the head and body of the embryo to the level of the 

 vitelline veins. The exceptional distance between these veins and the vitelline arteries in 

 the penguin embryo at this stage is one of the most remarkable diff'erences between it 

 and the fowl embryo of the same stage. 



3. Stage comparable with a fowl of 67 hours' incubation (Plate II, fig. i). NT 18. 

 Length 10-4 mm., measured as though the embryo was straightened out. 



Head flexed at right angles to the body, brain regions — hemispheres, optic lobes, and 

 medulla oblongata — clearly distinguishable. Choroid fissure open. Otocyst still in con- 

 tact with the surface ectoderm. Olfactory pit. Number of mesoderm segments 39. 



Pharynx distorted by visceral clefts (c). Of these, numbers I and II perforate the 

 ectoderm, numbers III and IV lie close to the surface but do not penetrate it. The hind- 

 gut is completed. 



Rudiments of (i) the liver, and (ii) the lung. The liver rudiments arise as two pouches, 

 one on either side of the main venous trunk as it enters the heart. The lung rudiment, 

 arising as a constriction on the floor of the pharynx, is already divided at its hinder end 

 into two small pouches. These come behind the last visceral cleft and extend nearly to 

 the level of the apex of the heart. As they are simply prolongations of the enteric wall 

 their cavities are lined with endoderm. Owing possibly to the position of the embryo, 

 with its left side towards the mass of yolk, the left lung rudiment is retarded in its growth 

 in comparison with the right. This inequality is of course only temporary, and in the 

 later stages there is no obvious diflference in size between the two lungs. 



The heart lies within a closed pericardiac cavity. The ventricle and conus regions are 

 clearly demarcated on the right side. 



Limb rudiments have made their appearance: (/) fore-limb, [h) hind-limb. 



Amnion complete. At its hind end it is attached to the serous layer by a short con- 

 nective {a) which recalls the longer amniotic funnels figured by Schauinsland (1903) in 

 embryos of Piiffiiius, Phaeton, and Diornedea. 



Allantois rudimentary, just visible outside the body {at). 



4. Stage comparable with a fowl of 4 days 8 hours' incubation (Plate II, fig. 2). NT 25. 

 Length, measured along the dorsal surface from the top of the optic lobes to the tip 



of the tail, 20 mm. This is about 5 mm. longer than the corresponding stage in the fowl. 

 Brain regions with the exception of the cerebellum are all established. 

 Retinal pigment appearing in the eye, separation of the lens from the ectoderm. 

 Otocyst deeply embedded and only indicated at the surface by a shallow depression. 

 Olfactory pits wide open and forming spacious cavities. 



