REVIEW. 87 



derm which is turned towards the mother is raised up into an 

 oval area : at about the hinder third of this is the blastopore in 

 the form of a depression, semilunar at first, but soon becoming 

 horseshoe-shaped, and with its convexity turned towards what 

 will afterwards become the anterior extremity of the embryo. 

 Immediately in front of the blastopore is a perceptible swelling of 

 the blastoderm (a), marking the spot where the Juture thoracico- 

 abdominal process is destined to appear. 



A longitudinal section of the blastoderm at this stage (fig. 2, 

 woodcut) shows that the archenteron (a) is already a considerable 

 cavity, having its long axis directed forwards from the blastopore 

 {b), so that its front wall makes an acute angle with the anterior 

 part of the blastoderm. The endoderm cells (eji.) lining it are 

 columnar, as also are those of the ectoderm (ec.) covering the 

 embryonic area : the latter pass insensibly into the pavement 

 cells of the general surface of the blastoderm. 



The mesoderm has already appeared in the form of a number 

 of globular cells (ms.) confined, at present, almost entirely to 

 the central portion of the embryonic area, beneath the thoracico- 

 abdominal process and in front of the archenteron. 



In the second stage (PI. VI, fig. Z) the blastopore has under- 

 gone a notable change : from being horse-shoc-shaped it has 

 become irregularly circular, its ends having united behind, and 

 enclosed a raised plug-like area, the surface of which is at first on 

 the same level as that of the blastoderm generally. The archen- 

 teric cavity has increased considerably, and its distal end has 

 begun to curve forwards, or towards the head-end of the embryo. 

 (Pig. 3, woodcut.) 



In the third stage (PI. VI, fig. 3) far more important and 

 extensive changes have taken place. The embryonic area is no 

 longer oval, but has taken on a sort of rough heart- shape, being 

 divided anteriorly into two well-defined lobes, the procephalic 

 processes. The blastopore (g) has decreased greatly in trans- 

 verse diameter, and the endodermal plug (h), which in the last 

 stage nearly filled the aperture, has undergone a marked diminu- 

 tion in size, and has sunk so far below the line of the blastoderm 

 that the gastrula-mouth is converted into a definite oval opening. 



Extending from the depression between the procephalic lobes 

 to the anterior end of the abdominal elevation is a shallow 

 median furrow (r) ; this may be called the medullary groove, as 

 the central nervous system is subsequently produced from the 

 cells lining it. 



There is no evidence to show whether this groove is formed, 

 as in Amphioxus, from behind forwards : the existence of the 

 thoracico-abdominal process entirely prevents the possibility of 

 a communication between its posterior end and the blastopore. 



