BY J. P. HILL AND C. J. MARTIX. 739 



In the head region the cerebral vesicles are indicated by 

 widening of the medullary plate. In the region of the hind- 

 brain there are four pairs of well-marked neuromeres, and in the 

 anterior region of the mid-brain a single pair of neuromeres, much 

 less distinct than those of the hind-brain. Opposite the 2nd and 

 3rd neuromeres there exists on each side an extensive auditory 

 plate already slightly grooved. 



There are 17 pairs of somites which in the middle region of 

 the trunk possess distinct cavities. Just external to the outer 

 edges of the somites (with the exception of the first three) and 

 between them and the lateral zone of mesoderm thei'e occurs a 

 narrow intermediate zone containing the Anlage of the Wolfiian 

 Vjody. From the 7 th somite posteriorly the Anlage of the 

 Wolffian duct is free from the Anlage of the tubules. At its 

 posterior end the Anlage of the Wolffian duct becomes reduced 

 to a single cell which passes directly over into the ectodei'm. 



Double heart Anlagen are present, situated in greater part 

 opposite the hind-brain region. No trace of a vascular area was 

 visible in the fresh condition, though in the hardened blastoderm 

 developing vessels are indicated by a mottling both in and around 

 the amniotic area. 



A distinct blastopore is present. It leads into the blastoporic 

 canal which runs forward in the head process of the primitive 

 streak and opens into the cavity of the vesicle. 



The primitive streak extends behind the blastopore to a distance 

 of 1"5 mm. The embryo more nearly resembles that of the 

 Virginian Opossum {Didelphys) of 73 hours, described by Selenka, 

 than any other embryo known to the authors. The Platypus 

 embryo is, however, much longer. The marked retardation in 

 the formation of the medullary folds and in the folding of the 

 embryo is one of the most characteristic features of the embryo 

 at this stage. This may be due to the mechanical effect of the 

 rapid imbibition by the ovum of nutritive fluid secreted by the 

 uterine glands. 



This paper, with illustrations, Avill appear in the next Part of 

 the Proceedino's. 



