18 ARTHUR DENDY. 



rowed, so that the emhryo is now broadest in the middle 

 about where tlie lower limb of the head-fold joins the under- 

 lying blastoderm (fig. 7, F. Sp.). The hinder half of the 

 embryo is narrower than the broadest part of the head-fold, 

 and forms a well-defined elevation above the surrounding blas- 

 toderm, with the conspicuous blastopore {B. P.) lying in the 

 mid-dorsal line near the posterior end. The most posterior 

 portion of the embryo is formed by the primitive streak behind 

 the blastopore, which rises up, and is very sharply defined 

 from the surrounding blastoderm (fig. 7). 



In the mid-dorsal line of the embryo, between the blasto- 

 pore and the anterior extremity, a faint indication of the 

 medullary groove was visible even in the opaque, unstained 

 embryo (fig. 6, 31. G.). After staining and clearing the 

 medullary groove was more conspicuous (fig. 7, M. G.), but 

 owing to a certain amount of crumpling in the head-fold its 

 course could not be exactly followed except in sections. 



A selection from the series of transverse sections into which 

 this embryo was cut is represented in figs. 8 to 14, the sections 

 being arranged in order from in front backwards. 



Fig. 8 represents a section through the area pellucida, just 

 in front of the head-fold, and it will be seen at once that it 

 presents a very curious appearance. In and about the middle 

 line the three germinal layers are all clearly differentiated. 

 The epiblast {Ep.) lies above, composed of a single layer of short 

 columnar cells, with very large, darkly staining nuclei. The hypo- 

 blast {Hyp.) lies below, and is formed of short columnar cells 

 clearly derived from the lowermost cells of the original lower 

 layer. These hypoblast cells present an appearance of cilia- 

 tion, but this may be due to post-mortem changes. Between 

 the epiblast and hypoblast the remainder of the original lower- 

 layer cells form the mesoblast {Mes.), which has already split 

 into two perfectly distinct layers, an outer somatopleuric, Avhich 

 cleaves to the epiblast, and an inner splanchnopleuric, which 

 cleaves to the hypoblast. Between these two a wide cavity 

 {P. C.) has appeared, the development of which causes the 

 somatopleure to bulge slightly upwards, and the splanchno- 



